For my home, yup, always just call a plumber. Their hourly fee is not much, compared to my time, and all the trips back/forth to home center, trying to figure out, get what I need.
My FW has a Dometic 310, which I really like! I don't know what is under it, but I do know it is bolted through the 5/8 inch floor deck, and is very solid.
Good pics! Not sure I'd want to notch that circle, which may cause it to crack at some point. Could it just simply be bolted to the floor, without a ring, or the half rings pictured previous?
Jerry