Hey welcome to the fun of multiple drives. If you have the drives available then truenas (which uses OpenZFS) is better IMHO than hardware raid as it is just more resilient and you can (most of the time) move zfs data/pools around easily. I’m guessing ye olde Dell has a Dell raid card, and whilst they are ok, zfs is way better nowadays. If you value your data then parity and mirroring make for way better usage of drive space if you are careful. I mean zfs easily can support losing a whole drive (or 2 if you wish) without losing data, and the rebuild speed is pretty much 10X faster than old dell raid cards IF a drive does fail. Truenas has some great features built in for backup etc. and a decent truenas setup is way more flexible than a lot of the cheaper pre-built nas boxes as well. Of course, your mileage might vary, but I wouldn’t be without my truenas setup now. It also serves via shares to the android tv box for movies etc. I don’t even bother with plex. Whilst you might keep a single drive as a backup of the backup that pretty much wouldn’t be needed in zfs as the data is pretty damn secure with parity, but you might not agree. The other beauty of using zfs is that it is perfectly happy to use sata/sas/m.2 drives or whatever as long as they are accessed directly, though with an old dell raid card if it doesn’t support it mode or pass through for the drives it doesn’t work - zfs just needs simple access to the drive
to be fair I really just used my relay for myself and didn’t run into that many issues… but this IS the reality if you want to help others like you said…