Every software screen-time limit is disabled by the person who set it, immediately, because the same hands control both. A physical constraint changes who has to act to remove it. The design question is where the friction sits so that it survives the moment it is most wanted.
The open question. What restraint can a person not undo in three seconds?
What would kill it
People are extraordinarily good at circumventing constraints they installed themselves, so the winning version probably changes the environment rather than adding another lock. Commodity hardware with a subscription is not that.