I’ve been using Weechat-Android to connect to my self-hosted Weechat for over a decade. This is one of the killer mobile apps that keeps me on Android and I love it.
I also have a couple instances of thelounge that people use on mobile via the PWA (progressive web app).
I used to use VLC for music, but these days I use Symphony to play local files on my phone. VLC tended to struggle when scanning or indexing large folders (which it did all the time…), while Symphony is a bit better at that. That said, I still use VLC for video and for casting things from my DLNA server (VLC supports Chromecast).
For ebooks, I’ve used Librera FD and that has been mostly OK. I’ll checkout the two you mentioned though. Thanks!