There already is an option to set block times but as you mentioned if a user has the password, they can override the blocked times.
I think this is an easy feature to add to the current dashboard.
Such a feature is already on the list of features waiting for development.
You can vote it up so that it gets more traction and hopefully will be developed sooner.
Last I checked it was number 7 on the list.
https://netfree.link/app/#/tools/feature-request