I’m pretty happy with my Ubuntu, docker, and ZFS with Sanoid/Syncoid server. Nothing against NAS focused distros, I just haven’t come across a compelling enough reason to switch.
For hardware, I use a Odroid H4+. Intel N97, 4 SATA ports, Intel quick sync, and low power. It’s running my 20ish docker containers with plenty of power to spare. It’s been great
If you know iptables, just stick with that. In my testing, docker containers seem to ignore ufw rules. Supposedly, iptable rules are respected but I haven’t learned iptables yet so I can’t verify.