I also swapped out the stock seat torture device with a Cloud-9 Cruiser Select within a couple days of putting my Mini ST on the road. It's a big improvement but my butt is still somewhat sore after 30 minutes or so (it only took 4-5 minutes with the stock seat). So I have a Bikeroo Senior (
https://amzn.to/3bhjg7v) on order, due to arrive next week, just to see if it's a bit better. I think seats are like shoes; you have to try them on to see if they fit, and maybe I just haven't developed the right butt callouses yet.
The Cloud 9 is a bit heavy, but very well built and solid - to install, remove the included C9 post adapter and just mount its rails directly to the stock Rad Mini post.
UPDATE June 14: I've now tried the Bikeroo and a Velmia, both are well-made seats but just didn't improve things. After adding a Redshift suspension seatpost, I eventually settled on a different Cloud 9 seat, their Cruiser Gel Sofa model, which is wider and flatter than the Cruiser Select and for me the best of the bunch so far.
https://amzn.to/3helhU7 The nose on this one is about 1.5" too long IMHO, but doesn't get in the way. Unfortunately, you have to keep trying seats to find the best fit, but a good return policy for the rejects helps.
My butt still gets sore, not as much, but shifting around a bit helps and the Redshift really helps smooth out the bumps along the way.