I have had lots of problems with my butt in the last year or so which is more due to me having a accident over a year ago and has changed the way I sit on the bike, I have had bike and saddle fits which have relieved most of it. So if the op is having butt problems after 30 miles in my opinion its probably got something to do with the way you sit on the bike/saddle ( ok we tend to base opinions on our own experiences) . Never had a problem before even using the cheapet of shorts. I bought Assos bib's and shorts ( Halibu and F1 Mille) to help relieve my butt problems and yes the pad is significantly thicker than my gores, decathlon, castelli and vittesse club shorts/tights.
I am very happy with them and just got a brand new set of bib tights free off assos as they took too long to do my free crash repair ( result there they kept the damaged ones) .
Most significantly for me is I am a shortie the Assos kit fits me like a glove, the tights are the only ones I have that dont wrinkle like Nora Batty's tights and the shorts are the only ones that dont sit on top of my knee cap I actually have a gap to my knee.
So if the op is a tall person you might want to buy the longer leg length shorts.
In my experience ( only had them since xmas) I dont think they are going to be that long wearing, the seat area is quite shiny already on the tights as I wear them the most, maybe thats cos I move a lot on the saddle which is probably the seat of my problems anyway. Still need chamois cream to keep my butt pain free, but I did 107 miles last weekend in them completely pain free.