Nostream looks really good, well written and well maintained howerver it is a typescript project (javascript) what means it cannot use all the cpu cores to process messages.

Scaling by balancing load is somehow the standard way to go but at the extreme it is more expensive to scale i think.

I would like to write a relay but there are only a handful of language that seems to fit well.

A zig relay would be a nice project to start, at least to lern the language.