Others have answered most questions but I just wanted to point out a few things:
NAS is network attached storage: a separate server which makes ‘shares’/volumes available over the local network. A DAS is ‘direct attached storage’ which plugs directly into a machine. Since you have a server, a NAS is probably the right route. https://www.techradar.com/features/das-vs-nas-what-is-the-difference
Good that you mentioned both backups and redundancy as they’re not the same. https://www.tencentcloud.com/techpedia/108368
Some people want a dedicated machine for being a NAS, while others want to make use of the hardware by making it pull double-duty as a server. I have an old PC I loaded up with drives and installed truenas on: I made zfs pools (opposed to raid) and exposed shares to the network. I can set up virtual machines or use “plugins” / jails for hosting other services like immich etc. E.g. https://www.truenas.com/docs/solutions/integrations/nextcloud/
Regarding backup versioning, modern filesystems like zfs and brfs have snapshot features. Regarding “one backup”, it depends how important it is to you. https://www.backblaze.com/blog/the-3-2-1-backup-strategy/
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