Documents folder becomes inaccessible in the Finder

You should not be accessing it through /SSD - Data.

The fact that it is mounted means it is not the actual Data volume.

The actual Data volume is mounted at /System/Volumes/Data.

I imagine what you are seeing is an orphaned Data volume left behind when you reinstalled the OS.

If that is the currently in-use Data volume, something is seriously wrong.

Ok, same problem with a twist. You reinstalled onto the original Data Volume. The original startup volume is the orphan.

You should be able to remove SSD, but you likely have duplicate storage on the SSD - Data volume.

The easiest solution I can see is to erase the drive completely, reinstall, and migrate from your backup.

If you restored to the - Data volume, you could probably just remove the SSD volume. Maybe it will all work out once that is gone. I think a restore will erase the destination, so you likely don't have duplicate files on the startup volume.

My concern with a reinstall onto the Data volume is that it would would leave your data intact, maybe move it to Relocated Items.

Since you restored, I think it erases the volume, so you wouldn't have the old data.

What shows up in Disk Utility?

Are there two - Data volumes?

The Data volume should not be mounted at /Volumes.