If "outbox" works, can you not divide the people you're following into chunks and use separate connections to the same relay?
Discussion
We’re actually doing that now, but for some reason there are still issue we are trying to figure out. At some point it becomes a bandwidth issue regardless
Back in 2022 i had the idea of a custom query you would send to relays that was basically “give me all the notes whos authors are on my kind 3” no need to send your entire contact list each time, but its very ad hoc
This might be the current situation, but it doesn't seem fundamental to me. If there's a way to reliably reproduce these issues it should be straightforward to optimize