What size is the drain line from the toilet forward? Are you sure that there aren't any flat spots or bellies in the line? It must all slope down with at LEAST 1/4"/foot slope. My guess is that there's a partial blockage between the toilet and the main drain, and a proper snaking may solve the issue. Today's code would not allow 1.5" pipe for the shower, and in many places, the minimum size under a slab is 2", but what you have may have been code compliant when installed. An inside flange fitting on a 3" pipe isn't the ideal thing to use, but can work.