But as it was, he barely did enough damage with the FGT to trigger Urahara's plan. If he's waited until Aizen evolved to be almost as strong as him, it might not have even done that much.
Conversely, if he'd started the fight with the FGT instead of letting Aizen evolve twice first, it might have actually obliterated him and he wouldn't have even needed to rely on Urahara.
I agree it certainly wasn't the most tense final showdown as a result, but I think he actually avoided one of the big complaints people always have with Shounen manga. The way characters could easily win fights in one attack if they skipped right to their big attack, but characters always start with their weakest attacks and work their way up during a fight, until, their most powerful attack always ends up being their "desperate final attack".
To me, this felt like Ichigo being uncharacteristically genre savvy going, "You know what, this guy has already crossed the bishounen line like twice, I know nothing but my final attack is going to work, I'm just gonna skip to the end."