If I understand correctly you wish to fuck a woman as deeply as possible without causing her pain. I do understand. Although I love all cocks and I actually enjoy deep thrusting sometimes in the right position and with the right size cock it does hit the back of my vagina pounding on my cervix. It can be uncomfortable. I’ve learned we can change position to make it more comfortable and with less pounding on my cervix however I honestly don’t mind those super deep thrusts. It isn’t especially painful to me. Maybe afterwards I will have some rawness or even walk a little bow legged lol. But hell that happens after any long night of group sex with lots of different sized cocks. I guess I would suggest working with your partners in getting positions that allow deep penetration but that is also comfortable for them. A woman will let you know if you are doing it right as such.