Sounds like you might have some buggy / bad app interactions going on? There’s really no reason for a 12 Pro to be slow at this point even if the OS was getting “heavier.” The 12 Pros have 6GB of RAM, which is now pretty standard across the iPhone lineup even years later. And the A14 SoC in the 12 Pro is effectively the same tech as the M1 processors that are still rock-solid at running full blown macOS, albeit with fewer cores running at somewhat lower clocks.
I can’t really think of much reason for you to be having a slow experience aside from the usual bugs that can accompany any new major release, and usually get ironed out over the course of a month or two.
FWIW you also don’t have to update. Of course they bully you to update. You don’t have to.