Very cool! Which Linux distributions have you tried? Which one do you recommend for best hardware support (WLAN hotspot, audio, touch)?

The Shift6mq (sdm845 as OnePlus 6 and Pocophone F1) might be interesting for some users because they are still manufactured and hence used ones in good conditions can be found.

The experience for me is now more pleasant, because I can run it on devices that are still sold (on the first market) and one can even buy new devices with Ubuntu Touch preinstalled as dual boot with Android (Volla phones).

The default password is already quite short and changing it to just 1 character always worked for me. If you build your own pmOS, your choice of password seems to be unrestricted. How do you change your pasword?

There are several (paid) Matrix services that allow bridges to other networks like Signal, WhatsApp, Telegram etc. Then, you can use matrix clients like nheko or matrix’ web version in a browser,

I doubt it. The approaches of Ubuntu Touch and postmarketOS are too different.