Our sex life is, for lack of a better word, fairly hardcore. He somewhat regularly is the receiver of fisting, and we have recently talked about trying cervical sounding in a sexual context, but couldn't find any good safety info about it so decided to try it when we have the money for an urgent care/ER visit if something goes wrong

I know IUDs are safe and non-intrusive with more "vanilla" sexual activities (i.e., PIV, fingering, oral), but I'm curious about the safety with more intense play and haven't found good resources beyond a handful of folks on LJ in 2006 sharing that they didn't have issues, as long as they asked their provider to cut the strings a little shorter.
My questions are: Is fisting actually safe, or were these folks just lucky? If fisting is safe, is cervical stimulation (petting, prodding, poking the dimple) safe? I imagine cervical sounding would not be an option, but would love to be told I'm wrong. Any other thoughts/considerations about IUDs and harder sex acts would be greatly appreciated as well. Cheers!