I’m thicc and wear a 36 in most pants, i wear an XL in exofficios. They stretch a good amount without cutting into my thighs. It sounds like you are thinner than me but I have the same issue, my thighs and behind normally drive the size of whatever I’m wearing vs the waist. Too bad my biceps don’t drive the size of my shirts haha.
Edit to add:
Most of the merino underwear and even base layers are merino nylon blends, I had some merino poly blend baselayers that sucked, but I’ve never had a problem with any merino nylon or spandex blend. I don’t remember the poly percentage of the one I hated. I think the nylon makes it more durable. I have some old 100% merino first lite, seems to stretch a little easier than the newer blends. I’d bet the stone glacier undies are probably pretty good too just looking at the specs.