Too many faces are created in probuilder

Hello. I am new in probuilder. I just wanted to make some models and texture them. I started with arch, copied it and scaled, subtracted small arch from big in order to receive an arch door, but after boolean action, I received many many faces on my new door. How can I avoid this kind of problems? Because I don't want to apply material to each of 70-80 faces separately. Even when I merge faces wasting my time, I receive an ugly blurred object, and material fits imperfectly.
Please look at the first image. It has just several faces, easy to merge. How to get the second object as smooth as first?