1. Roll your own budget NAS
2. Find the cheapest dollar per terabyte storage you can
3. Profit

I think you’ll probably be installing something like plex on a custom built or a synology.

It’s more of a question of how much time you have and budget. Do you want something with nextcloud like functionality out of the box? Synology isn’t a bad choice. It has all the apps for cloud backups, quotas, users, etc.

Do you want to toy with your system more? Roll your own and choose the best software for your needs.

If you’re not going to get into proper raid/zfs storage pool type solutions, you could potential use lvm to span drives. You don’t mention what platform you’re doing this on.

Probably an adb backup with app data.

edit files onto it

You might want to clarify what this means, because it is confusing. Are you going to edit files on the NAS or not?

I don’t have any personal experience with asustor. Seems decently-regarded, and probably is a good 1st NAS in terms of user-friendliness.

They support 2.5gbps speeds, so you may want to consider making sure whatever you edit on can do 2.5gbps (and your switch) as well to take advantage.