Seriously - over time, the cushions and support structure of the seat will change and "soften" up. It's the nature of foam.
It may be just a situation of changing the way you sit in the seat - move a bit more to one side or the other? If you have a power seat adjustment, you could (in theory) add a gel pad or some other type of cushion to sit on, but this would also impact your left side and if that's fine, then ...
Question - which side do you carry your wallet on? I'd suggest taking your wallet out of the back pocket when driving and see if that helps.
The cushions and the support structure (in the old days - springs) will be pretty much the same left to right, no one side being firmer than the other. But time will (as with all things in life) sag and squish that foam cushion and/or the support structure. Short of having a custom molded seat cushion, there's not much to do.
But try sitting a bit more "this way or that way" and see if that helps. If you're using cruise control, shift where your leg rests... If you're not using cruise control, start using it - see if that helps. And if you have your wallet in your right back pocket (as most guys often do) pull it out when you're driving and put it in a jacket pocket or something and see if that helps. Just don't leave it in the car when you get out of the car...
I hear you!
Being somewhat lacking in 'natural rear end padding' I had to resort to a sheet of memory foam under the seat covers of my previous Forester. In my case, that solved the comfort issue.
Friends with the same vehicle also noted the firmness, so it's not just us.
Thankfully, my 2020 is much improved in this area. I use nothing but neoprene seat covers now and it's just fine.
I find my 2020 forester sport seat bottom feels fine. However, I wish it has more bucket support. I feel it is a little too flat and wide. However, my wife’s crosstrek bucket seat is deeper and hug my butt just fine.