Hello and sorry your tank is smelling so badly! I keep freshwater tanks, have for a very long time, and in my opinion your TopFin filter doesn't exactly suck, but it's not one of the better filters, and I don't think it will keep up with messy goldfish. I'd suggest an AquaClear Hang On Back filter or one of the Seachem Tidal HOB filters. You may even want two with goldfish.
How old is the tank? Is this a well established tank where lots and lots of detritus has accumulated in the substrate? If so, this can be a source of the poo smell.
If large water changes and gravel vacs are not helping, then whatever it is must still be in the tank. Personally, since these are goldfish in only a 40 gallon, I'd just reboot the whole tank. I'd put the fish and snails in a bucket or tub, drain the whole tank, removed and rinse/clean/replace the sand or gravel and decorations, scrub the tank and filters real good, refill and restart it. Add dechlor and nitrifying bacteria as needed for a new tank. If your smell comes back after that, your goldfish have some serious problems!
Good luck!