I understand many found Mojave stable, but I don't know if it was designed specifically with a focus on "under the hood" improvements. If you go to
https://en.wikipedia.org/wiki/MacOS_version_history and search for "hood", you'll find only two OS's that (at least unofficially) are identified as focusing on the latter: Snow Leopard and and High Sierra.
Likely the reason I didn't have the teething pains you did with HS is because I nearly always wait unit about x.3 or x.4 before installing a new OS. That's not just to allow time for Apple time to straighten out their issues; it's to allow for the app developers to do this as well.
What have you found about the performance of Big Sur vs. Catalina? I know there were a lot of complaints about Catalina, but maybe all the significant issues were fixed by 10.15.7.