I've always gone with a hotswap enclosure connected to an old raspberry pi - or whatever computer I can scavenge - over USB. Probably not the most performant but I don't use them as network mounts so its w/e.

Regarding mix-matched drives: just extend them manually with LVM and don't fuss with anything like zfs or RAID. If you've got an SSD in there you can use it as a cache with LVM.