Don’t worry, Plasma 6 will default to half square and hat rounded instead.

Unraid 6.12 and higher has full support for ZFS pools. You can even use ZFS in the Unraid Array itself - allowing you to use many, but not all, of ZFS extended features. Self healing isn’t one of those features, though, it would be incompatible with Unraid’s parity approach to data integrity.

I just changed my cache pool from BTRFS to ZFS with Raid 1 and encryption, it was a breeze.

I generally recommend TrueNAS for projects where speed and security are more important than anything else and Unraid where (hard- and software-)flexibility, power efficiency, ease of use and a very extensive and healthy ecosystem are more pressing concerns.

Plex supports native auto deletion, just like you are asking for. For Jellyfin there’s a plug-in: https://github.com/terrelsa13/MUMC

Plus for some odd reason the layout on my shieldTV is an absolute disaster for some reason

That’s because Jellyfin’s clients are still mostly terrible. Jellyfin is the more flexible media server, Plex has the far, far better clients.