Basically, you want to run a messaging service over an onion routing network (Tor, I2P), or even better, a mix network. You should check out Nym (https://nymtech.net/) and come back with what you think about it. It is very suitable to what you want, and Loopix is resistant to global adversaries. https://arxiv.org/abs/1703.00536 (Loopix's paper)