Read this:
https://www.lifewire.com/move-macs-home-folder-new-location-2260157 I know you said you did this, but follow the steps very carefully and it should work. Note that the "/" mark is key in the path. In post #5 you said you changed to "Volumes/{path}/{username}" but left off the "/" that needs to be in front of Volumes to start the path at the root.
What you are trying to do is risky, but that article says how to do it. The one thing I would NOT do is the Verify section, Step 7. Instead of deleting the original home directory, I would just transfer the contents of that folder to the trash and leave the folder there. There are some apps that will default to that location even if the home folder is clearly indicated elsewhere, and if it's not there, they will crash. So leave the old home folder there and just periodically check to see if anything has been put there and you can identify what app put it there.
Your system will be slower with this arrangement as most of your read and writes are to the home folder, which is now going to be on the slower spinner drive.
What I think happened is that somehow the system's fusion drive got un-fused.
BTW, the reason you can't see those files on that other drive is that it's not yours, according to the system security. Security is very strong on macOS. I know that YOU think that the owner of the files is you, but the system says you own what is in your Home directory and little else. Those files you moved to the spinner are, to the system, someone else's files. Get the process done right and you will be the owner of those files, but maybe not the ones in the old Home folder. If you end up in that condition, come back, there are some ways to sort things out.
But let's get the Home directory over first and see.