I've been using ZFS on my Linux servers for years and have pretty good experience with it. I've not been following Linux development too much. Is bcachefs usable/stable/reliable enough to replace it?
There's a NAS appliance called NASty and it shares publicly usage stats: https://nasty-telemetry.pages.dev/ Those numbers are NASty alone. There are more users on other systems.
Sometimes starting fresh with one coherent codebase and all features design baked in from the start might be better.