I’d honestly choose a similar stack for the back-end. I have limited experience with Rust, but my impressions so far is that it’s a language that allows you to make changes with confidence that they’ll work. I feel like starting something in Rust is somewhat difficult, but contributing is relatively simple.
For the front-end, I don’t think the choice is as important, since I think that by virtue of being federated and being able to have multiple front-ends, it would almost be better for the front-end to be managed by other parts of the community. And I say that as a primarily front-end/developer-experience dev.
I would probably default to React since I’m familiar with it and it’s very popular, but would probably be tempted to experiment with something better.
Easiest solution IMO if you’re already using CloudFlare, even the free version: you can filter out certain countries. Otherwise, there’s probably other alternatives, even open source ones. Good terms to search for with your cloud provider or self hosted software may be middlewares, firewalls, serverless, edge functions, etc.