Without knowing what ypu plan is in detail, here’s one example of a plan for a NAS…

• Flash your SAS card or get an LSI card you can flash to IT mode.
• Install TrueNAS Scale and set up your ZFS volume with your existing SAS drives
• If any drive fails, exchange it for a SATA with at least the same size and re-silver.

You wouldn’t need to exchange all of them at the same time as long as the one you are swapping in can hold all the blocks the old one did.

Yeah, your data is taken hostage. When cancelling the subscription it’s a good idea to delete everything manually before the account expires. Even if you can’t guarantee Dropbox haven’t just flagged the files as deleted, it signals intention. I did this when migrating to pCloud and haven’t received any reminders from Dropbox.