The OS is still unstable
sounds like you have a shitty TV. I have had 3 smart TVs of different brands and price range going back 10 years and I can't think of a single time I've noticed any OS issues, because I literally never use the OS.
tries to apply a bunch of filters that need to be disabled
This has nothing to do with smart features, any decent TV should have image processing options. You will have better picture and motion quality (especially on LCD/OLED) with a properly configured TV than you will with a TV/monitor without any of it. Sure lots of options are not needed, but I wouldn't say having options for how to do that is a bad thing.
has extreme lag unless gaming mode is being used
Yes, if you want zero lag, you turn on gaming mode. I'm not even sure what your complaint is here other than "having options is bad".
and has stupid UI decisions like putting the audio level exactly where the subtitles usually are so that changing audio will obfuscate them.
Also has nothing to do with the "smart" functions, the exact same thing could happen with a poorly designed dumb TV or monitor as well.
Once every 24h I‘m also getting a warning that the tv is not connected to the internet, despite network connectivity being explicitly disabled.
again, this is not a thing for most smart TVs, you just have a shitty one.