So, I have a Staff Manager, and I would like a feature or permission node to deny him to modification of my user and group. I can deny modification of my group, Owner, just not my specific user. Could you use a UUID possibly?

I'd say that you can make a group that inherits from Owner group and with the permissions you need, then make yourself inherit from this group, then deny modification of this group. It's a workaround but still.

This is what I'd suggest too ^^ :)