There are peer to peer (or otherwise decentralised, e.g. multiple servers operated by many different people) alternatives to Signal. I haven't tried any of them, nor do I know which ones have been audited. But if the worst comes to the worst, there are alternatives that are hard to block.

In particular I remember reading about DeltaChat which piggiebacks on normal email infrastructure, making it really hard to block without massive collateral damage.

Been using matrix for years now. Even operating a server for a small community. It is slightly more involved for regular users though so that is not the best but otherwise works. Setting up your own federated instance is quite simple, there is an Ansible playbook around to make it even easier. You could even isolate it and have it only for your org. From what I know there is some traction to get it adopted in a bunch of agencies eu-wide.