Protecting a folder is not that difficult...
You said that you think the shared users have admin rights.
If you merely change permissions on the folder, then anyone with admin access (in a couple of steps) can also easily get access to files on the same shared system.
If you encrypt the folder, then no one can open the folder without the unlocking password.
You are (gently) misled if you think that is "too complex". It's really not that hard, nor does it take a lot of time.
OR, even simpler --- keep your folder somewhere else (not on the same shared drive.)
If you are keeping your personal records (that you don't want anyone else to read) on the same drive, then sharing your drive to others with admin rights is a bad idea.
If your folder holds "private entertainment", then all the more reason to keep it on a completely different volume (and one that you should unmount/disconnect when it is not in use.
If you do not choose to protect your folder, either through encryption/password protection, and absolutely cannot move it to another drive, then you should re-think your decision to allow admin access to your personal drive.
Just my opinion: Keep the files you want to share on a network shared volume of some kind (not on your booting system)