Okay so first there was Unix. It was semi Open Source and a bunch of companies were making different versions that were becoming increasingly incompatible. That is why POSIX was created, it standardizes major parts of Unix. Linux is a Unix like operating system, meaning it functions similarly but doesn’t share any code. One thing that POSIX standardizes is the shell meaning there’s a standard how a loop works etc. Most shell on Linux like bash and zsh are POSIX compliant but some (like fish aren’t). This means a command that works one way in bash might work differently in fish. Basic stuff is mostly the same in my experience so if you’re not having any problems you shouldn’t worry about being POSIX compliant. If you want most of the same stuff but POSIX compliant checkout zsh. Fish provides documentation for adjusting your commands so I’d just ignore it until you run into a problem and then take a look at the docks
If you(they) don’t mind having outdated software Debian stable might be worth looking into. Otherwise there are immutable distros which are very hard to fuck up, and even if you do there is the option to rollback to the previous version. I’d recommend Aurora or Fedora (fedora doesn’t include some proprietary stuff like some codecs so if you need that it’s probably better to use Aurora).
Linux Mint also has a version based on Debian stable, LMDE, which is could also be an option. It’s not as stable as Debian as it adds its own stuff but has the out of the box experience.
As general advice I’d suggest using less packages and more flatpaks as a faulty flatpak update can only break that flatpak, not your system. For packages be sure to disable online updates, meaning you have to reboot to apply them. This isn’t as convenient but if stability is that important to you I’d go for it