Unofficial crDroid with Poco X2. No real issues, although I haven't been able to get SBC-XQ working which it supposedly supports.

Stock android ROM(s) sound like a good fit since you seem to like a minimal experience.

GrapheneOS would probably suit you too and has improved privacy and security, but it's Pixel only so I've never got to try it myself.

It's not trivial but not as hard as I thought, at least last time I tried it. The absurd amount of disk space the build environment requires, along with the time to actually clone it it, is probably the most annoying part.

You could use it as a source for contributing links rather than interacting with existing threads. Which is more important in the early days, particularly for niche communities.

There's also audiobookshelf for a self-hosted approach to audiodooks and podcasts, although the podcast functionality does still need some work.

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.

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/

LineageOS seems fine for your use case (and it has official support), but Pixel users are honestly spoilt for choice on custom ROMs so you could try out a few. Maybe Pixel Experience if you want to keep the Google "look", or crdroid for another AOSP rom.