Had the same issue. I have a VM at a hoster which proxies requests to my nextcloud server at home. Both the VM and my server on my home network are connected via tailscale. I’ve been using the VM for other stuff as well and happened to have it anyways, I didn’t get one just for this purpose

There are some that do. Inoreader as host and Reeder as client both support that. Not perfect, but working well enough

I’ve been pretty happy with by Pocketbook so far

Yep, that’s the whole point of Nebula

Not yet, because it hasn’t officially released yet. Will happen soon(ish) though I think. Some mastodon clients work with pixelfed as well, although not perfectly.