We use Mailgun as outbound relay for self-hosted email stack, and no more problem with delivery. As a home user you can even stay within the free tier and never pay a dollar for it.
In the Android app, switch from one player to another, from internal to the web player in the settings. It solves the 5.1 playback issue. (Of course, it introduces others, eg. manual screen brightness but it's not as a big issue as no sound.)
I've tried several docks and cables with no success until I've found this article: https://www.bigmessowires.com/2019/05/19/explaining-4k-60hz-... The recommended CableMatters dock solved the 4k60Hz issue with USB2.0 pheripherals connected (mouse, kb).
A simple thing won me over XMPP to Matrix: .well-known domain delegation to use @asd:example.com as username instead of @asd:some.subdomain.example.com while being able to use the subdomain cert for the server instead of the parent domain cert. XMPP only supported SRV delegation which required the parent cert on the virtual (sub)domain.
Maybe now it's time to refresh the research papers on Matrix/Riot privacy concerns. After reading these a few months back, I wasn't so sure anymore that Matrix/Riot is what I can wholehartedly trust: https://github.com/libremonde-org/paper-research-privacy-mat...