Short answer
No. Do not wait. Everyone sins, Muslims too, and the shahada is not a promise never to sin again. It is the declaration that there is no god but Allah (﷾) and that Muhammad (ﷺ) is His messenger. Islam wipes out the sins that came before it. The Prophet (ﷺ) said that if people did not sin, Allah (﷾) would bring people who sin, ask His forgiveness and are forgiven. No one knows when or where he will die. The Qur'an warns that turning back only when death comes is too late. So take your shahada now, then leave your sins one step at a time. Even wine was forbidden in stages.
Full explanation
Everyone sins
The Prophet (ﷺ) swore that if people did not sin at all, Allah (﷾) would take them away and bring a people who sin, then ask His forgiveness, and He would forgive them. The point is not that sin is good. It is that Allah (﷾) made people who fall short and turn back to Him, and He loves to forgive them. The Qur'an praises believers who do something shameful or wrong their own souls, then remember Allah (﷾) at once and ask Him to forgive them. They do not insist on the wrong. Such people are still believers. They have simply sinned and turned back.
Abu Hurayra reports that the Prophet (ﷺ) swore by the One in whose hand his life is: if you did not sin, Allah would take you away and bring a people who sin, then ask Allah's forgiveness, and He would forgive them.
“And those who, having done something to be ashamed of, or wronged their own souls, earnestly bring Allah to mind, and ask for forgiveness for their sins,- and who can forgive sins except Allah?- and are never obstinate in persisting knowingly in (the wrong) they have done.”
The shahada gives you a clean page
When Amr ibn al-As came to accept Islam, he held his hand back from the pledge until he was sure his past would be forgiven. The Prophet (ﷺ) asked him whether he did not know that Islam wipes out all that came before it. The Qur'an promises that Allah (﷾) changes the evil deeds of those who repent, believe and do good into good deeds. It tells everyone who has wronged himself not to despair of Allah's (﷾) mercy, for He forgives all sins. A new Muslim starts with nothing against him.
Amr ibn al-As, near death, said that before Islam no one was more hateful to him than the Prophet (ﷺ), and he had wished to kill him. He told how he came to the Prophet to accept Islam. He asked the Prophet to stretch out his right hand so he could pledge, then drew his own hand back. Asked why, he said he wanted a condition: that he be forgiven. The Prophet asked whether he did not know that Islam wipes out everything that came before it.
“Those who invoke not, with Allah, any other god, nor slay such life as Allah has made sacred except for just cause, nor commit fornication; - and any that does this (not only) meets punishment. (But) the Penalty on the Day of Judgment will be doubled to him, and he will dwell therein in ignominy,- Unless he repents, believes, and works righteous deeds, for Allah will change the evil of such persons into good, and Allah is Oft-Forgiving, Most Merciful,”
“Say: "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.”
Waiting has its own danger
No one knows what he will earn tomorrow, or in what land he will die. The Qur'an says Allah (﷾) accepts the repentance of those who do wrong and turn back soon after. It says the repentance of one who keeps on doing wrong until death comes, and only then says "Now have I repented", is of no effect. A person who waits to be free of every sin first may never reach that day. The safer path is to declare faith now, while there is time.
“Verily the knowledge of the Hour is with Allah (alone). It is He Who sends down rain, and He Who knows what is in the wombs. Nor does any one know what it is that he will earn on the morrow: Nor does any one know in what land he is to die. Verily with Allah is full knowledge and He is acquainted (with all things).”
“Allah accept the repentance of those who do evil in ignorance and repent soon afterwards; to them will Allah turn in mercy: For Allah is full of knowledge and wisdom. Of no effect is the repentance of those who continue to do evil, until death faces one of them, and he says, "Now have I repented indeed;" nor of those who die rejecting Faith: for them have We prepared a punishment most grievous.”
Even wine was forbidden in stages
Allah (﷾) did not forbid wine all at once. First the Qur'an said that in wine and gambling there is great sin and some profit, but the sin is greater. Then it told believers not to come to prayer with a clouded mind. Last it called intoxicants Satan's handiwork and told believers to keep away from them. Aisha, the Prophet's (ﷺ) wife, explained why. The first parts of the Qur'an spoke of Paradise and Hell. Only when people had taken Islam to heart did the rulings on lawful and unlawful come. If the first thing revealed had been "Do not drink wine", she said, people would have answered that they would never leave it.
“They ask thee concerning wine and gambling. Say: "In them is great sin, and some profit, for men; but the sin is greater than the profit." They ask thee how much they are to spend; Say: "What is beyond your needs." Thus doth Allah Make clear to you His Signs: In order that ye may consider-”
“O ye who believe! Approach not prayers with a mind befogged, until ye can understand all that ye say,- nor in a state of ceremonial impurity (Except when travelling on the road), until after washing your whole body. If ye are ill, or on a journey, or one of you cometh from offices of nature, or ye have been in contact with women, and ye find no water, then take for yourselves clean sand or earth, and rub therewith your faces and hands. For Allah doth blot out sins and forgive again and again.”
“O ye who believe! Intoxicants and gambling, (dedication of) stones, and (divination by) arrows, are an abomination,- of Satan's handwork: eschew such (abomination), that ye may prosper. Satan's plan is (but) to excite enmity and hatred between you, with intoxicants and gambling, and hinder you from the remembrance of Allah, and from prayer: will ye not then abstain?”
Aisha explains the order of revelation: the first passages spoke of Paradise and the Fire; only when people had come to Islam were the rulings on the lawful and the unlawful revealed. Had the first command been to stop drinking wine or committing adultery, she says, people would have refused to give them up.
Allah (﷾) asks only what you can bear
The Qur'an says Allah (﷾) places on no soul a burden greater than it can bear. It teaches the believer to pray for forgiveness when he forgets or falls into error. It also says good deeds remove bad ones, and it ties this to the daily prayers. So a new Muslim does not need to be perfect on the first day. He prays, does what good he can, asks forgiveness when he slips, and leaves what is wrong one step at a time.
“On no soul doth Allah Place a burden greater than it can bear. It gets every good that it earns, and it suffers every ill that it earns. (Pray:) "Our Lord! Condemn us not if we forget or fall into error; our Lord! Lay not on us a burden Like that which Thou didst lay on those before us; Our Lord! Lay not on us a burden greater than we have strength to bear. Blot out our sins, and grant us forgiveness. Have mercy on us. Thou art our Protector; Help us against those who stand against faith."”
“And establish regular prayers at the two ends of the day and at the approaches of the night: For those things, that are good remove those that are evil: Be that the word of remembrance to those who remember (their Lord):”
The usual worries
Qur'an 3:135 — Surat Al 'Imran (The Family of Imraan), verse 135
“And those who, having done something to be ashamed of, or wronged their own souls, earnestly bring Allah to mind, and ask for forgiveness for their sins,- and who can forgive sins except Allah?- and are never obstinate in persisting knowingly in (the wrong) they have done.”
Cited for
I will only sin again after my shahada, so I would be a hypocrite.
The response
A hypocrite pretends to believe. You believe, and you are worried about your sins. That worry is a sign of faith. The Qur'an praises believers who wrong their own souls, then remember Allah (﷾) and ask His forgiveness, and do not insist on the wrong.
Where it lands — Sinning after the shahada does not make a believer a hypocrite. Turning back to Allah (﷾) is what He asks.
Qur'an 31:34 — Surat Luqman (Luqman), verse 34
“Verily the knowledge of the Hour is with Allah (alone). It is He Who sends down rain, and He Who knows what is in the wombs. Nor does any one know what it is that he will earn on the morrow: Nor does any one know in what land he is to die. Verily with Allah is full knowledge and He is acquainted (with all things).”
Cited for
I will take my shahada once I have sorted my life out.
The response
No one knows when or where he will die, and repentance put off until death is of no effect. Islam wipes out the past the moment you accept it, so the shahada itself is the fresh start you are waiting for.
Where it lands — The shahada is the start of sorting your life out, not the reward for it.
Qur'an 5:90-91 — Surat Al-Maaida (The Table), verses 90-91
“O ye who believe! Intoxicants and gambling, (dedication of) stones, and (divination by) arrows, are an abomination,- of Satan's handwork: eschew such (abomination), that ye may prosper. Satan's plan is (but) to excite enmity and hatred between you, with intoxicants and gambling, and hinder you from the remembrance of Allah, and from prayer: will ye not then abstain?”
Cited for
I cannot give up drinking all at once.
The response
Allah (﷾) forbade wine in stages, and Aisha said people would have refused a sudden ban at the start. Take the shahada, keep your prayers, and work to leave it. Allah (﷾) places on no soul more than it can bear.
Where it lands — A habit you are still fighting is a reason to turn to Allah (﷾) now, not a reason to wait.
Perspectives
The Islamic perspective
The shahada is a declaration of faith, not a promise never to sin. Islam wipes out the past, Allah (﷾) forgives those who turn back to Him, and no one knows when he will die. So the shahada should not be put off.
The Prophet (ﷺ) spoke of what would happen if people did not sin. Allah (﷾) would bring a people who sin, ask His forgiveness and are forgiven.
The Prophet (ﷺ) told Amr ibn al-As that Islam wipes out everything that came before it.
The Qur'an says repentance put off until death comes is of no effect.
Wine was forbidden in three stages, and Aisha said a sudden ban at the start would have been refused.
Allah (﷾) places on no soul a burden greater than it can bear, and good deeds remove bad ones.