i am admin but i can do [/lp user DiamondTheo parent set Owner] and it works with pex you can't go to a group higher than you are, you can't promote yourself

well, don't give people that you don't fully trust luckperms access.

help

If you want to prevent people in lower groups from promoting themselves, you need to setup argument based permissions for LP commands.

More information on that can be found here.
https://github.com/lucko/LuckPerms/wiki/Argument-based-command-permissions