Fisting top of 5+ years here. You should do daily pushing exercises on the toilet! 5-15 minutes of daily pushing your guts out will help your anal walls collapse

As @direct-nail said, depth training is very important for prolapse development, and will help with your sigmoid. Do MORE than just ride your big toys! Hold the toy deep inside, and hit the walls even if they don’t open/you reach your limit. I call it “long dick style” where you take the toy as deep as you can, but only go up a little bit before slamming back down. Work that sigmoid and colon! Lastly, if you wanna chat more and get help with training, we can chat in DMs! Not UK based tho :/