I don’t ride dressage, but Free Ride breeches are essentially shapewear. I love them, as do many of my barnmates, my trainer, and my BO’s sister. They come in knee patch and full seat. Before I discovered them, I tried shapewear base layers under a traditional breech and was VERY uncomfortable.
ETA: despite being “compressive”, I find them very comfortable. I am still in my breeches 2 hours after getting home and they feel like leggings. But they shape, and smooth out all the bits. I do know they are, or were, a bit long for super short folks. But for most everyone else seem to fit fine - whether you’re average height or an Amazon like me (over 5’10”).