The flower doesn't blossom, a little gizmo on his prosthetic projects an image of a flower. Particularly, the one associated with the girl he tried to rescue and was ultimately only able to offer a mercy kill. If we're talking symbolism that's a significant difference. The idea I got there was that he was finding peace by reminding himself of why he did it all. It's been a while since I saw it but I didn't see much implication of humanity being reborn; putting aside themes and symbolism the city was the last remaining pocket of flesh-and-blood humans(I think?) and everybody there was either dead or had been converted into a pod person, slowly dying due to their mechanical bodies being out of power. The experiment to ensure human survival through forced evolution was a complete failure.
There was certainly a cathartic element to the protagonist dying on his own terms and being satisfied with his end but with a setting as inherently bleak as that there's just no way you're going to get anything that qualifies as a feel-good ending, at least not how I'd define it.
How I interpret Texhnolyze is like an inversion of the Plato's Cave Allegory. The surface is somehow more depressing than the hellish underground and at the end he finds his own light or a reminder of what he did it for like you say. The blooming flower of the ending prior to the finale is symbolic of a few things to me. Played with a beautiful song by GACKT, and it's one of my favorite ending themes ever. The sages proclaim Lukuss aka Lux will be reborn, and while the show doesn't disprove them they are eventually killed. But they seem important enough for me to trust their word.
It takes from Mouse Utopia too a little. I find it to be a far more satisfying ending than Lain's ending in terms of closure, it definitely feels better to me. I'm the minority when it comes to Texhnolyze. A depressing tone doesn't have to mean it's a feel bad ending, maybe I'm crazy but it was heartwarming to me and a little sad. The characters all died according to their values and stuck by them to the end. It's pretty inspiring, while the world is bleak (some) people are still human.
What makes it satisfying for me is the way it ties everything together by the end for those who decided to stick with it. Most people wouldn't last a 10 episodes in. It's a very rewarding anime and there's no other that have made me feel multiple emotions about it all at once.
TLDR is the best way to define the show is "hauntingly beautiful". There is beauty and joy in some tales of sorrow. I rejoice in it.
This song among many other tracks in the show represent it best.
(Instrumental version I found)
(Normal version with lyrics)
For better Texhnolyze analysis there's Crit Bonus's old YouTube series on it. He finally uploaded another entry after like 7 years lol. And then there's this for faster reading:
