You’ve clearly done your homework, and you’ve gotten a lot of good feedback already, so I’ll just add a few points…

• Encryption: If you’re going to bother with encryption, I wouldn’t half-ass it. Why bother at all if you’re fine using auto-decryption or a password that will be guess with any sizeable effort? Just lock it down with a strong password and decrypt/mount the data drive after any reboot; making a shell alias or script for this is trivial. You’re likely not rebooting the server more than once a week anyway.

• Budget/Specs: I get the sense you don’t have much budget right now, but knowing your hardware would help in suggesting solutions. Do you have an NVMe slot? What kind of mini-PC case?

• Filesystem: For simple storage, this really doesn’t matter and Ext4 will probably be fine. It’s a mature, robust, no-frills filesystem which is perfect for bulk file storage (docs, music, videos, etc.), but Btrfs would be fine too if you want more options.

• USB Docking Stations: I’ve had really good experiences with USB docking stations like this one, and I currently use it for attaching my backup HDDs each month. I wouldn’t want to rely on them for realtime data storage, but they do work wonderfully for backups and one-off drive access.