I’ve been going through a similar journey, and I’ll tell you want I did:

If you want open source NAS software, then TrueNAS and OpenMediaVault are the main option. Truenas has the better pedigree afaict, but it has pretty significant requirements that mean you’ll need expensive hardware. In the end, I decided it was way more than what I needed, I wanted my NAS to be purely a NAS, and I’d do my server/cluster on different hardware.
I almost got a HexOS NAS (fork of trueNAS SCALE with a front-end written by a bunch of ex-unraid folks to be much easier to configure and admin), but it’s still beta and I didn’t wanna wait a few months for GA, and also it has the same requirements as trueNAS, so it’d be expensive and you also have to pay for a license.
I ended up just getting a low-end 2 bay Synology NAS, because it is cheap, and easy to set up shares and backups, and 12tb mirrored is all I needed. I was too intimidated by the prospect of configuring trueNAS correctly, and Synology walked back their requirement of using their own branded drives.

If you go with a traditional OTS NAS, then you want raid 1 for a 2 bay or raid 5 if you have 4+ bays.
If you get something like truenas that uses ZFS then you want raidz1 (which is like raid5 with one parity disk). Current there are limitations with raiz if you wanna expand it later, but HexOS folks are sponsoring a ZFS feature called Any RAID, to make expanding raidz more flexible, which will presumably make it’s way to all ZFS NASes when it is finished.

I’m pretty early in my self-hosting journey, but so far I have a 2 bay Synology with cloud backup and a couple of shared volumes, a rasppi 5 running home assistant, a beelink ser5 running Ubuntu server for portainer, and a cheap VPS for pangolin.