I actually noticed on the custom rom that it only ran on V12.5 firmware when Miui is on V14 firmware which I think added APTX Adaptive and other things
You can normally update the firmware independently (or just flash stock then go back to custom after) as long as the custom ROM doesn't replace it. See: https://xiaomifirmwareupdater.com/
Honestly AOSP (stock Android) isn't that different to Google's look, just more minimalistic and most roms will be based on AOSP with some minor tweaks. You could look into custom launchers, although not really an area I'm familiar with.
Agreed on notifications, I like to block the majority of them, which you can mostly do with modern Android versions. You might like to use F-Droid if you haven't already, since all apps are open source and not monetary focused, so tend to have less obnoxiousness to them as well.