You’re going to want to look into something like Tailscale to get your own vpn going for remote access for backups. For a media server Plex just increased lifetime subscription prices to something crazy like $750 USD - might want to look into jellyfin.
iGPU should be alright for transcoding, but I enjoy having a dedicated GPU for tone mapping.
You got 6 cores, so should be enough to toss 1 or two at a VM.
General advice? If you run low on sata ports, use an HBA. Don’t use sata port multipliers, USB adapters, or expansion cards. A used HBA flashed to IT mode will likely be cheaper & will 100% save you headache when something goes wrong (slap a fan on it; like directly ontop of it).
Uhh what else?.. Run memtests, stress tests on the cpu, find the guide to burn in your HDDs before you put any data on them (this’ll take a WHILE, ~7 days for 8TB drives), only after you confirm your hardware is working should you make a pool & start putting data on it.
Start slow, figure out permissions & then how to get Plex working, then figure out how to make a VM & a network bridge (I’m assuming you want your VM to be able to access files on the NAS - if not skip the bridge), then slowly start deploying Tailscale, carefully test, slowly start setting up access for your friends to do their remote backups etc.
There is a lot of good content in the “Resources” tab - expect to spend the first week reading & running stress tests on the system.
A UPS is a nice to have.
Ignore any kind of of “cache” for your pools - unless you understand what L2ARC, Special vdev, or SLOG are, when you need them, if you need them, and how to manage risks. It isn’t a simple “throw hardware to increase HDD performance”. Without relevant use case, at best they’ll do nothing, realistically they can degraded performance, at worst they put your data at risk unless you’ve planned your configuration appropriately.
No need to make mirror boot pools unless you want to for fun; backing up your config file periodically will be more than enough to quickly restore your server if your boot drive ever dies. It is much more painless than you’d expect.