Hey Mike I'm very interested in Mosaic/next-gen nostr and would be happy to help out as much as I can. I'm still getting up to speed on nostr but I'm trying to build a client that has flexible Reddit and Discord style groups with modifiable private/public status that don't use relay-based groups. I was studying the problem of how to build this around NIP-EE and came across your work on NIP-A0 for better multi-device support.
It sounds like the nice UX problems I'm running into with nostr are similar to what led you to start working on Mosaic and it looks awesome so far!