After twenty years I’m not a Mac/Apple newbie either. I have to disagree again — sorry. What you see as “constant change” is the natural adaption to current needs of users. Maintaining software is expensive — we do want fixed bugs and we do want new features. Using iMovie from 2006 doesn’t help with new features and fixing bugs in a 15 year old complex software that was written to run on much different hardware for problems that changed (new formats, new needed features…) doesn’t really help. The extent of work the developers have to invest to support your decision to use grandpa iMovie instead of its younger, current form takes from the capability to support the 99,99% users who don’t identificate as software archeologists ;). Being the Indiana Jones of the software world is not necessarily a good thing.
I also disagree, that the changes Apple made are generally not good ones. I’m running Big Sur and to me it is the best macOS and right on track into a good future. Any quirks that are there will get worked on… yep thats again change and will again need me to install updates instead of stopping and complaining.