The power of Social Media is the community. Coupling the UI with Rust seems like it would prevent the larger community from contributing. I'm interested in both web and Rust, but have zero interest learning a Rust JSX variant.
Why not static site? Could have a themes folder where admins could drop their static themes. Also, would allow admins to host markup and Lemmy API on different hosts.
Probably want to keep services with different life cycles in separate docker compose files to allow you to shutdown/restart/reconfigure then separately. If containers depend on each other, then combining into compose file makes sense.
That said, experimenting is part of the fun, nothing wrong with testing it out and seeing if you like it.