AFAIK you can enable the snapcast server and then use an app like SnapCast to stream to your mobile devices.

I also use Posteo, one thing to note though is that Posteo doesn’t (and probably won’t any time soon) support custom domains. If that doesn’t bother you, it’s a great choice.

The other alternative I found during my research, which doesn’t have that limitation, is mailbox.org.

That’s incorrect, Graphene OS has Android Auto support.

If you have to access wikis from fandom, you can use breezewiki