PS, while I was closing the dozens of tabs that I opened to investigate how everything fits together, a note on what I wrote earlier:

this doesn’t mean that once mounted, the disk will be integrated in the ADM ui (you’re not going to be able to see it in the “External Devices” ui, nor be able to select it as a destination in the “Backup & Restore” ui).

mounting the disk on a path already accessible by ADM file explorer doesn’t work because of permission issues, similar to the immutable flag that you can see with lsattr… but someone on Reddit had another workaround:

https://www.reddit.com/r/asustor/comments/1bk7qsz/comment/kwrkeof/

I mounted the opened device to another path already accessible by ADM file explorer. I don’t think it will matter where really. But in my case I made two partitions on an external USB drive. The first partition is a small exFat formatted (10GB). The second partition takes up the rest of the drive and is formatted with cryptsetup (LUKS). Finally, to use this I open the LUKS device and mount it to a location in the first partition (which is automatically mounted by ADM)

In case others might want to find their profile on XHS (like I just did): https://www.xiaohongshu.com/user/profile/5c487dfa000000001003e19d