Openmediavault + mergerfs + snapraid is very similar to unraid storage in that you can add different disk sizes just like unraid. Admittedly it’s not as ‘plug and play’ as unraid, but it’s free, so can’t really complain. Disk speeds using this config are also much faster if that matters.

I have considered truneas for if/when I need to rebuild, but this works for my jellfyin/arr stack needs.