Panache sports bra. Hands down. I wear sports bras all day every day for work, and this one is comfortable enough to do that. The 2 separate cups mean no uni-boob (and besides looks, that means less boob sweat).
It has a converter to go from straight straps to racerback, and I’ll hook that for shows or rough horses or lots of sitting trot work. But I find the regular straps fine for most riding. The racerback setting will encourage shoulders back a bit, but not as much as something made for that.
They hold up well. I have 3 that I’ve been rotating daily for almost 3 years and they all look great. I machine wash gentle and hang to dry.
As a 28G, I understand the struggle.
ETA: Good every day bras will help your daily posture too, which helps strengthen those muscles for the saddle. So don’t skimp on those either.