I'm not aware of an enclosure that has a problem running with open slots. I have a Synology unit that's running with 3/8 empty bays.

I can speak to the Synology RS1221+ as I own two. They are great NASs but they are terrible at compute/transcode.

You missed an important detail which is how much data do you need to store?

Those big servers are nice and powerful if you can stomach the power bill.

My current setup is the two Synology's in an HA pair for storage with a pair of similar spec Hyve servers for compute. If you're looking for an all in one unit, the big server is the way to go.

Yes that's normal. Until the computer knows what decryption key to use with that specific disk, the disk is basically gibberish. In a sense that's good because anything you put on there is largely very private but bad because it's a bit of an inconvenience and losing the decryption key means losing the data on the disk. Up to you.