Hashem brings a flood to destroy the evil-doers (Braishis 7 (17)).
Without any notable change in the behaviour of mankind, and after Noach's sacrifice, Hashem decides that He will not repeat the destruction (Braishis 8 (21)).
How do we understand the decision process?
You claim that there is no noticeable change in human behavior after the flood but that is not true.
We don’t find the world, post flood, was rampant with bestiality or stealing.
I once read that one of the major sins of the Dor Hamabul is that by giving into their animalistic nature so much they lost all semblance of humanity.
But that was Noach’s favor that God found in him, that he was, as rashi says, someone who did Maasim Tovim - good deeds. This implies that the generation around him lost that basic sense of humanity and basic goodness completely
However, as opposed to the generation after the flood, though they wicked in their own ways, they at least had humanity within, or the ability to band together for a common purpose, even an evil one i.e. to build the tower to make war with God.
See here for this type of approach
Rav Hirsch says on Noach 8:21 that this shows that this was a dedication of the entire creation to fulfilling Hashem's Will. As a result, it removed the curse that Hashem had placed on the Earth and showed that even if people failed in the attempt, the entire creation would not be lost. This is like Hashem promising not to wipe out all of Bnai Yisrael, because no matter how bad things get, the bechirah chofshis means that there will always be some people worthy of being saved.
וַיָּ֣רַח יְהֹוָה֘ אֶת־רֵ֣יחַ הַנִּיחֹ֒חַ֒ וַיֹּ֨אמֶר יְהֹוָ֜ה אֶ
expression of compliance.ל־לִבּ֗וֹ לֹ֣א אֹ֠סִ֠ף לְקַלֵּ֨ל ע֤וֹד אֶת־הָֽאֲדָמָה֙ בַּֽעֲב֣וּר הָֽאָדָ֔ם כִּ֠י יֵ֣צֶר לֵ֧ב הָֽאָדָ֛ם רַ֖ע מִנְּעֻרָ֑יו וְלֹֽא־אֹסִ֥ף ע֛וֹד לְהַכּ֥וֹת אֶת־כָּל־חַ֖י כַּֽאֲשֶׁ֥ר עָשִֽׂיתִי:
Rav Hirsch translates אֶת־רֵ֣יחַ הַנִּיחֹ֒חַ֒ as expression of compliance
But Noach had the whole of future mankind before him, his offering referred to all that which, in the course of time, was to be accomplished by the whole of mankind to the final complete satisfaction of hashem with Man, hence his offering was ריח הניחוח, an indication of the complete compliance with Hashem's Will to which the whole of Mankind is to mature on the freshly regained Earth, now dedicated to be an altar of Hashem.
Rav Hirsch also addresses כִּ֠י יֵ֣צֶר לֵ֧ב הָֽאָדָ֛ם רַ֖ע מִנְּעֻרָ֑יו and translates it not as because but as even if.
when the images formed by the heart of Man from his youth, neither will I again smite any more every living thing
That is, even if the youth are dedicated to evil (so that there may be a reason to do this again), Hashem will not destroy the world again as the next generation in its rebellion against their fathers will go against the evil (as the time for each generation had decreased) because the world can be redeemed and would continue in the normal course of its cycles.
But as long as the bad ones reached their seven to eight hundred years, a better youth did not get a chance. In this new shortness of life, Hashem can make a generation die out quickly and a better one grow up in its place. Since then, mankind can ever look forward, and hope, and the future belong to youth.
...
If we add to this diversity of individuals, the still greater diversity of nations, which the new arrangements for the earth effected, and also the natural hindrance to communication brought about by the division of the earth into continents and countries which will only be (is being, I.L.) slowly overcome after thousands of years; if we think how, thereby, for thousands of years no degeneration spread universally over the world, and how, just as in the quicker change of the generations in the individuals, so thereby the wider development of the history of nations was started in which nenations with fresh unvitiated powers always take theplace of degenerated enervated ones: then, with this verse, everything is said by which Hashem started a completely new phase in the development and education of mankind. "If", says Hashem, "the endeavors of mankind are bad, and even from youth are devoted to evil, I will not, -- as I have done before -- for Man's own sake disturb the earth, and bring about universal destruction, but will rather arrange that all the contrasts of the times of days and years henceforth shall be present and working simultaneously on the earth".
The Akedat Yitshak (B'reshit Sha'ar 14) explains that the sacrifice that Noah offered was not the direct reason for God to promise not to destroy Man. Rather, it was indicative of a permanent change in the nature of mankind, such that God knew (not decided) that he would not destroy them:
אמר אל לבו לא אוסיף לקללם כי גלוי וידוע לפניו טבע תכונות האדם ומה שקבלו בזה מההכנעה וההרתעה הטבעית והמוסר המוטבע ביצירתם עד שיספיק זה לשלא יבאו עוד להפסד כולל כמו הראשונים אשר אחזו מעשי אבותיהם בידיהם ונמחו מעל פני האדמה. ולזה ידע כי לא יקלל עוד את האדמה בכלל בעבור האדם
See further there how he explains the seeming conflicting reason given; that Man's inclination is evil from his youth.
This is similarly the implication of Radak who writes (Genesis 8:21)
...כי ידע כי לא יהיה בעולם עוד כמו אותו הדור ויאמר ה' אל לבו - ראה בשכלו שלא ישחית עוד את האדמה כי לא יהיה עוד רעים רבים, ויוסרו מדור המבול שיהיה קבלה בידם דור אחר דור
God knew that mankind would never return to its antediluvian state, and that there would be a tradition of the flood that would be transmitted to future generations to keep them in check. That is, God did not change his mind or decide (in the classical sense) not to destroy Man. Rather, circumstances changed, and given the circumstances, God knew that he would no longer need to destroy man.
Tangentially, regarding the seeming contradiction in reasons to not destroy Man, with verse 21 indicating a different reason; Man's bad nature, Rav Saadya Gaon (there) understands the verse differently.
לֹֽא־אֹ֠סִף לְקַלֵּ֨ל ע֤וֹד אֶת־הָֽאֲדָמָה֙ בַּעֲב֣וּר הָֽאָדָ֔ם כִּ֠י יֵ֣צֶר לֵ֧ב הָאָדָ֛ם רַ֖ע מִנְּעֻרָ֑יו
It doesn't mean that the reason that God would not destroy man is that he is bad natured, rather it means that God would not destroy Man over his bad nature. That is, it is saying that in spite of Man's nature, God would not destroy him.
