I want a server running nextcloud, immich and others.

I have a N100 mini server with a 2TB external HDD. I want to secure the system against data loss. Hence, I want a backup and redundancy.

1. Most important question: How do I build everything? Is this a NAS? My naive approach is to buy 3 external HDDs and connect them to the N100 with a USB hub. I assume this is not "the right way" but to use/build a NAS. Do I have to build a separate NAS computer? When I lookup NAS buying, it is a computer with a case for 4 drives, excluding the drives and costs 400 bucks. I am confused because this is incredibly expensive compared to what I already have. What is the additional benefit compared to my setup? Am I cheap?

2. Regarding redundancy, is RAID still the way to go? At 2 TB, using RAID 5 with 3 drives sounds good. I'd have 4 TB of usable space, much more than I intend to use in the next years, and adding a drive increases the storage by 2 TB, effectively increasing space by 50%.

3. I have 4 TB usable space, but I won't reach 2 TB in the next one or two years. I'd use a 2 TB HDD for a local backup via borg. Once my hot storage needs to increase, I replace the backup drive with a larger one and use it to increase the RAID storage. Is one backup sufficient? Or should I keeping multiple versions of the data. Daily, weekly, monthly backups? What is your experience with it?

4. Another 2 TB HDD for an offsite backup, LUKS encrypted, backed up once a year (that's the goal for now).

Does that sound good?