If the only goal is an open source file server (we call this a SAM-SD, there is a section of the forum just for that) then the most likely recommendations for an OS will be openSuse Tumbleweed or Fedora. CentOS and Ubuntu are fine choices too. FreeBSD is excellent, but less well known.
You make it sound as though the wise choice here would be to install OpenSuse etc direct to the hardware.
So in a "read back" mode, this tells us that the data stored here isn't important so there isn't really anything to worry about. If you do the expansion and it causes data loss, they can't be upset as they don't see value in the data. This also tells us that the servers shouldn't be there, because if they aren't backed up and they aren't a caching system then you shouldn't have them at all.
Thanks - I will proceed without using ZFS - I prefer hardware RAID.
ZFS is perfectly fine with hardware RAID, if you like ZFS' features otherwise (like zsend is nice) then there's no reason to avoid it. If you don't plan to use any unique features, then XFS is my "go to" choice by default. Very fast, very stable.