I love undies shopping! And there's nothing wrong with a $20ish target bra. I like their
playtex t-shirt bras a lot for regular t-shirt wear.
To me you sound like you are in the wrong size both in the band and the cup with all the sag problems.
Measure yourself under the bust. Round to even.
This is your band size. 80-90% of the bra support is in the band. Only 10-20% is in the straps.
DO NOT be adding extra inches even if things online tell you to. That's old advice from before modern elastic fabrics. If it rides up in the back you are in too big a band size or tightening up the straps too much trying to lift the girls.
When you put it on, it should be snug on the largest set of hooks. Then as the bra ages, you can hook it closer to make up for the elastic sag as the bra ages. Bras need to be changed
every 6 mos or so. We wear these things daily -- they work hard!
For cup size, measure yourself around the fullest part of the bust and subtract band size.
0 diffrence = A cup
1 inch = B cup
2 inch = C cup
3 inch = D cup
4 inch = DD cup
(go here if you need more cups sizes. Towards middle of page) That ought to get you in the ball park for when you go try on.
Your breasts should fill out the cups nicely and smoothly with no extra fabric bunching (cup too big) or extra breast spilling out the sides or tops of the cups (cup too small). Bras are in so many cuts to accommodate cuts in clothes -- plunge, demi, full coverage, etc. So once you are in the ball park, take a close look at the cups actually ON your body. And if it is a special bra for a special outfit, bring the dress to try it all on together!
If you are a big C or D or larger, somewhere around there is time to consider underwire or rope or similar support. The wire shouldn't be poking you at the sides. It should lay flat on your ribs.
And you know how to put on a bra right? Bend over so the girls point to the floor. Put the bra on and fasten the hook and then straighten up. Then the girls are centered.
Or just put it on and then reach in there on each side to get the girls up and centered in each of their cups.
HTH!
A.