A long narrow dildo would help for depth training, yes. Another thing that helps is trying different positions (like on laying on your back vs on you side vs squatting on the toy, sometimes while keeping the toy inside) to see what's most comfortable for you. A stool is also a helpful aid for squatting. Also very helpful is (counterintuitively) pushing out with your butt/guts (as if you were sitting on the toilet) while pushing the toy in (or sitting on the toy), because your insides o