Any recommendations?
XMPP is very efficient and delightfully simple to use and administrate. I never tried E2EC with it, but there are options on the clients, like OMEMO. It has limited federation.
The simplest server IMO is Openfire, but offers range from Lua-written Prosody to the extremely expensive Isode servers that can do complete federation, HF radio XMPP and probably coffee and pizzas too.
But turns out this milk is from the Asian water buffalo, which is even bigger than the African kind, but can be domesticated.