Well, you may need to change out your seat... and you did that... so you might find some relief there.
I know for me, if I don't ride 2-4 days a week... when I get back on my bike, my butt is sore for days.
It's like no matter how thick the padding in the shorts are... my boney sit bones just jab into the saddle. MISERABLE.
I need a nice racing saddle, with EXTRA PADDING where my sit bones hit. I'm thinking about creating something myself. I saw where someone used some mouse pads and duck tape... and it seemed to work!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"