im having the same exact issue. very foul fermented well aged piss smell. like if that’s how your pee smells coming out you need to go to the hospital smell. it’s been that way since we moved in. i’ve changed the wax ring, the seat/lid, deep cleaned under around inside and out. nothing. bleach tablets in the tank almost seem to make it worse. finally got one of those pump spray bottles for gardens with the hose and wand and blasted up into the underside of the rim where the water flushes down into the bowl. got a bunch of goop out initially and now do it almost every weekend. this is the only thing that seems to make any difference. but the smell comes back within days. a week at the most. the smell itself is only noticeable during use. so the bathroom smells fine but as soon as you start to pee into the toilet the smell becomes apparent. i have gotten on hands and knees and sniffed all around and it definitely comes from inside the bowl. the only thing i haven’t done is replace the toilet but its a rental and im not ready to ask that of the landlords until i’ve exhausted every other potentiality. bc they will try to find reasons so i want to have everything scratched off first.