>In general, you want to get the fastest boot drive you can.
Pretty much all NAS like operation systems run in memory, so in general you're better off running the OS from some shitty 128gb sata ssd and using the nvme for data/cache/similar where it actually matters. Some OS are even happy to use a usb stick but that only works for OS designed to accommodate this (unraid I think does). Something like proxmox would destroy the stick.
Also, on HDDs - worth reading up on SMR drives before buying. And these days considering an all flash build if you don't have TBs of content