Of course, it is legitimate to terminate the collaboration if you are not satisfied.
But the question is: what about the service provided up to this point?
That must be compensated, the architect does not work for free!
Service phase 1 according to HOAI = basic evaluation
Service phase 2 according to HOAI = preliminary design (rough sketches!) and initial cost estimate
He has delivered that according to your statements, hasn’t he? You have received a preliminary draft and a cost estimate, haven’t you!?
As a construction expert, I see little chance of getting out of this situation.
You should try to settle amicably with the architect!
Best regards!
Goldbeere