I've just emerged from OS-X upgrade hell myself.
My wife's 2019 MacBook Pro 16 inch became increasingly unstable under Sonoma and finally stopped booting altogether. In the face of a £650 quote from the Apple Store she decided to replace it with the latest M3 and we took the old one home to see if I could salvage anything.
To cut a very long story short: after several dead ends, I finally got Catalina up, running and stable. I think that if I stay away from operating system upgrades, this will serve me for another ten years. By the way, I strongly suggest that you keep a full clone of your bare bones system, once it's stable. That way, if anything goes wrong, you should be able to rebuild from scratch.
I hope the guys in charge will forgive me for plugging Carbon Copy Cloner at this point - it really is your best friend. Make a bootable copy of your system before it starts getting clogged with updates and at least you can go back to a stable version.