I’m not sure that I fully understand your post either, but presumably you are not the only Owner that’s having problems with the ‘bad’ Strata Manager, so why not place a Motion on the Agenda of that Plan’s next General Meeting to terminate the Owners Corporation’s Agency Agreement with them, and to enter into a new Agreement with the ‘good’ Strata Manager?
In the meantime, have a chat with the ‘good’ Strata Manager about their willingness to take on the additional Plan, and if they are (willing) then seek their advice about how best to terminate the Agreement with the ‘bad’ Strata Manager.
…and liddle lamzy divey.