Good idea, I support this.
I would have preferred to make scope the master tag of the two others, hence I started with the proposal of making off-topic a synonym for scope.
Then I tried the same with on-topic-definition. This lead to an error message saying this wasn't possible,since there are currently more than 1.25 questions tagged with on-topic-definition than with scope.
So my next try was to make on-topic-definition the master tag for scope as well as for off-topic. Unfortunately, both of those attempts failed, because of the pending vote off-topic -> scope.
Maybe I screwed this up, and the system does not offer me any way to retract my first proposal (note this was a feature request from 2010, but it seems SE's staff did not implement this in the last 13 years). Maybe we just have to wait now until the voting is over, maybe we need help from a diamond mod or from the SE staff, I don't know. I guess I will recheck the status in a few weeks.
Here is the voting page of scope,
To all of you with voting privileges for tag synonyms: it would be really nice if you could cast a downvote for this accidentally proposed synonym "off-topic".
Then we can start a new proposal to make scope and off-topic synonyms for the master on-topic-definition.