I tried Google Pixel 3 It works, but hardware is really weak and especially battery.

PMOS doesn’t manage energy compensation well. I have two FP 5 (Stock Android and PMOS) can’t tell the difference but it noticeable more energy consumption on PMOS side.

I’m really happy with FP 5, but sound doesn’t work. Need to use headphones.

Would it be a part of plasma mobile?

I found it useful, maybe it helps you too https://wiki.postmarketos.org/wiki/Fairphone_5_(fairphone-fp5)/Audio#How_to_get_involved

Mainlining, getting started page is quite good for start.

True. I had app from Flubhub