Selfhosted

Yeah. That's what I used to do when I started out.

The simplest thing to do is install Debian on the computer and create partitions. You have 4 HDDs and 2 SSDs so it'd be stupid to create 6 separate partitions for each drive.

See in the BIOS if your motherboard supports software RAID1, so you are protected against drive failure somewhat. This will allow you to get something barebones running that'll use at least 2 drives with redundancy. I assume the mobo RAID1 is stupid and only allows for max 2 drives, so the other drives will be just laying around useless. If that's the case, probably use the 2 SSDs first. I see other posters recommending higher orders of RAID, but I only have 2 HDDs so I never really delved into that :P Perhaps that's sound

With a system like that you could probably set up some small NFS for sharing your files by configuring it manually from the terminal.

Note that going with raw linux is "simpler" in the sense that it's perhaps easier to wrap your head around or tinker with, but TrueNAS or Unraid have GUIs that will allow you to create e.g. the mentioned NFS share with a few clicks, rather than having to do it from the terminal. Depends on what you're looking for. You could move up to TrueNAS or Unraid once you've played with raw Linux enough for example.

Once you have that,

I only ever dealt with ZFS and TrueNAS. ZFS will allow you to create a "partition" (pool in zfs terms) from many drives at the same time, so you'd be able to use more drives than just the two from RAID1.

The drives that you have are probably shitty SMR drives whose write speed dramatically slows down once you're writing to them for a longer time. Consider buying CMR drives in the future, or just going all-SSD if it fits your usecase. ZFS hates SMR drives.