I guess you're right. To be honest, the post-movie seasons and the early HD seasons all kind of blur together and either way, it was around that time I fully realized The Simpsons went to shit.
I'd still tune in for the annual Treehouse of Horror for a few years after that, and there are a small handful of Zombie Simpsons episodes I actually kind of like (like the one where they go to Ireland) but otherwise the show was totally unsalvageable after the movie, and it spent the three or four seasons before the movie in a weird state of "meh" where it wasn't as good as the Golden Age seasons or the Silver Age/Scully Era seasons but it wasn't quite bad enough to be Zombie Simpsons.
In retrospect, I think the show fully became Zombie Simpsons from Season 18 to Season 20, with the movie being the closest thing to a single benchmark moment.
Before that, there's the Golden Age (Seasons 1-8 or Seasons 1-10) and the Silver Age (Seasons 9-13 or Seasons 11-13), the mediocrity era (Seasons 14-17) and then Zombie Simpsons (Season 18 onward)