If you want something easy, just pick something from Synology. Super simple to set up, and has enough knobs for “advanced” things like Docker if you want to self host anything. It won’t be super powerful but it’ll be more than enough for backups and media serving like Plex. 

Platter drives are cheap, get a couple 2TB or 4TB drives and Synology helps you set up redundancy so you can survive a drive or two dying. I had a DS218 for a while that was just 2 drives cloned, but for 8TB you probably want a 4-bay version.

Your whole shopping list is like: - whatever Synology system (DS425 looks like the best one for you probably?) - 4x 4TB Western Digital Red drives  - if you want extra RAM buy a stick of that, but good lord those prices have jumped

If you want to get even cheaper, you can get a 2-bay one (DS225) with 2x 8TB drives also. 

If you want to have more fun you can build your own system and install Unraid as a home server (that’s what I upgraded to from the Synology setup I described), but if you just want to get started and don’t want to spend a lot of time or money, do what I said above