It might just be the legging materials. Are you wearing high (or some form of) compression leggings for working out or ones advertised as “buttery”? Mine are mostly advertised as for compression and stretch to some degree especially if I’m doing something dynamic like climbing and hiking, but they’re back to normal after washing and drying. I’ve had the best luck with 80% polyester and 20% elastane/spandex/lycra. I feel like anything with more than 20% of elastane provides too much stretch if that makes sense. The ones I wear for working out/hiking feel thick and substantial and just a touch uncomfortable when you first put it on.

If I’m doing squats or articulating my knees a lot, I like to bunch a bit of excess fabric around my knees for more comfort as I don’t like the feel of stretching fabric in that area. But I’ve never had creases stay after washing, like they’ll be there while worn-in, but gone once washed and dried. I can see the stretching you’re mentioning in regards to more comfort-focus leggings. But I usually stick to high compression for workouts, so that may be the differentiating factor.

I’ve also not had that issue of leggings creasing (butt or knees) even when there’s a seam there, but that might be do the compression, thicker fabric, and the fact that I’m probably less curvy than you. But if you’re feeling self-conscience about it you could wear shorts with no liners on top. With the added benefit that if you sit on rougher surfaces it doesn’t fuzz up your leggings. These days though, I prefer pants/joggers over leggings for a majority of things.

I don’t own any sheer tights (and haven’t for a very long time now) so I can’t quite compare how different wearing those would be in comparison to leggings. I just stick a leg in, pull up past my calf, repeat on the other side, pull up the rest once they’re comfortably past my calves, and adjust around the knees give them a bit more flex room.