Sorry I only know this from things I’ve read myself so it’s at best third hand info, but yeah from what I understand F-Droid is pretty much under constant threat of just vanishing.
I use quite a lot of F-Droid apps myself so obviously I don’t want it to and wish it had an alternative.
An observation I’ve made using bubblupnp on my Android phone is I can’t directly cast to my fire cube with my vpn app on yet if I open vlc on the cube itself it can see the dlna library and play it.
So yeah it’s a bit strange.