Yes, relayFilters stream approach was something that I came up with the outbox in mind, it's been very useful and easy to split filters of a single subscription, I also designed in a way to work batching multiple unrelated subscriptions together, everything becomes a queue at the end consumed by the start() operators, this approach has been working really well for me and haven't touch much in a while.

rx-nostr is indeed too tightly coupled and when I started my mind was mainly focus on the batcher.

The way I been designing things is the core the be completely stateless and lazy, you can create a subscription and the core will never initiate any subscription for you, just like rxjs itself wouldn't, it just gives you the building blocks.