if you want em headless, you might wanna try dietpi (https://dietpi.com/). It's a debian base with an active developer maintaining a bunch of very useful utilities designed to make managing pis and SBCs as simple as possible. Better yet, it's designed to be extremely minimal on system resources, and strips out stuff like the display server unless you choose to install it.
Ive had it running on a homeserver on a pi4b for years now and it's rock solid.
None yet. But comparisons ive seen (such as snapraid's website) suggested it was a limitation, but they could be quite old, still.
That's great to know, though. So no weird plugin management just using ZFS? Or would I need MergerFS in that scenario?