I will respond even though this post is several days old because I actually do this. I have some vpses on Hetzner that run Silverblue no problem. It is not an install option available by default there, but support uploaded an iso under my account quickly when asked.

If you do it, change the active firewalld zone. The default is for a desktop, so not great for vps space.

Librera Reader for everything. I use it for both books on a phone and comics on a tablet—covers both great! Plus it is on F-Droid.

Yep, except with http here (python3 -m uploadserver).

There is also Rocket.chat, to complete the list, though it is supposed to be difficult to host.

I would recommend Matrix. My small instance is running Conduit, and it works great!