I've been modifying Model 3 / Y seats for a year and a half now. I finally bought my first Model X used seat to explore. I just started looking it over. It was listed as a 2021-2023 seat (listings are not always accurate). The one I happened to buy is white and perforated. The following things I notice so far. The seat is fundamentally the same as in the Model 3 / Y. There are some differences but the similarities are important for now. The width in the seat bottom towards the rear is the exact same width as the Model 3 /Y. This makes sense because they all share the same metal frame base. This leads to pain in the butt! I've had several Model X owners contact me to see if I could help them with their pain. I had to turn them away because I hadn't done any work with the Model X as yet. I plan to modify my first Model X seat soon, now that I have one. The goals are to improve the support and suspension of the seat bottom. To eliminate pain and provide for a seat that can be sat in for many hours pain free. To provide genuine suspension so that bumps in the road don't get sent through the back. This all looks possible. I notice also that the seat sensor is identical but the other wiring is different from Model 3 / Y. The foam appears to be attached to the seat Tesla OEM seat cover in the same way. Nothing that I see so far should hinder my foam replacement, while retaining all the original electricals. I don't have a method to retain the perforations through the replacement more robust foam I use. I don't know how important that will be to Model X owners.
I should add that I don't understand where and how the original poster's seatbelt was hurting them. I've never heard of that issue before this. It was stated at the hip, so that must be the "female" part of the buckle where you insert the upper strap into. I'll have to think more as I've never heard of this complaint before. I would think my added seat cushion would raise the seating position relative to the belt insert, so that might be improved also.