Scholars_Mate

I've been using FolderSync. It let's you sync to/from a remote server through a bunch of different protocols like SMB, SFTP, and WebDAV. You can schedule it to run automatically.

I haven't tried it myself, but I think there is a way you can get Tasker to run scripts in Termux. You could set up your own rsync scripts that way.

If you’re worried enough to encrypt the drive, you shouldn’t be auto mounting it.

This really depends on your threat model. If you are only concerned about the drive getting stolen, or wanting to keep the data on it private if you need to RMA the drive, mounting it automatically on boot with a key stored on the rootfs can be perfectly fine. If you are a journalist in a hostile country and protecting your sources from state level actors is a matter of life and death, then yeah, this would be woefully insufficient.

It can be. Some of those keys might just taking advantage of regional pricing to buy keys in cheaper markets and resell them elsewhere, but some of them are purchased using stolen credit cards. Those actually cost the devs money from chargeback fees once the actually owner of the credit cards finds the fraudulent transactions. It's hard to say how many of them are from stolen credit cards, and key-resellers try to stop it, but it still happens. This was back in 2019, but the Factorio devs had a couple blog posts talking about G2A:

Blog post saying G2A is worse than piracy

Follow up blog post where G2A reimburses them (at very bottom)