I would go for SSDs if I needed speed. SSDs longevity is just fine. Any drive can die when you leave it unused for a decade.
Pvt-Snafu
Well, I'd say that a single device can't be both easily accessible and secure. A NAS like Synology wold be my choice for simple and convenient access and then another backup to B2 with rclone.
Both. Or HDD/SSD plus cloud like Backblaze Personal or Wasabi. The main thing here is to have several backup copies. Any drive can fail unexpectedly.
I understand it doesn't have to be a NAS but it would be much better in my opinion. RAID enclosure over USB is...well, not the most reliable solution. I would look into some Synology options. As has quite nice DSM and it can combine drives of different size in the SHR RAID: https://kb.synology.com/en-af/DSM/tutorial/What_is_Synology_Hybrid_RAID_SHR
I honestly don't see a huge need for a NAS in your case. Especially, if you still have free space on the HDDs. I mean, a single HDD can cover your needs easily and I don't see any benefits in a NAS for backups.
I don't think it matters that much. It's more important to have several backup copies and follow the 3-2-1 rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/. For backups, I would just go with a cheaper option.
I would add that flash drives are also less reliable. If you store some important data there, I would go for an SSD.
ZFS can do self-healing if you drives are in mirror or RAIDZ1/2/3. I would put those two drives in ZFS mirror. Then you would scale it by adding another two drives in mirror.
ZFS doen't need much RAM. All that talk about 32GB RAM or 1GB per TB storage is nonsense. It will work with any amount of RAM. Primarily, ZFS needs more RAM if you're using deduplication.
Keep several backup copies. HDD plus SSD and keep one drive in some remote place. Or SSD plus cloud like Backblaze B2. Overall, follow the 3-2-1 backup rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/
SD cards are not the most reliable media in general. I would just go with an SSD at this point.
Well, I would say, keep multiple backup copies. An external drive plus cloud like Backblaze B2 which I think is exactly your case in terms of price. Upload encrypted with rclone.
No problem, mate:)