Hello and thanks for sharing your unique self made macerator project - very interesting what us RV'ers can do when we take a notion to do so. Concerning your thoughts on the flange, you're right the pump should be supported in some manner so its not bouncing around hanging on the flange. On my Leisure RV. I supported it with a large hose clamp to the body, where on my Renegade, I mounted a hard foam block under the pump to support its weight.
While I'm here I mentioned the Clean Dump hose set - mine arrived and was sent back the same day the hose is so stiff it was not even un-coilable and the fitting took two hands to turn, this hose is not designed for RV use and would be near impossible to store especially in cold weather.
I highly recommend that you install the Threrford macerator 10 ft or 20 foot hose since it is very flexible and stores easily. I also included a shot of my water bay hose compartment which was re-purposed to hold my water hose. As I mentioned I dislike any sewer activity in the water bay for health reasons.
Stay Safe - Mike