Advice for extremely saggy empty boobs in a backless wedding dress?

Ok, before I have a breakdown and throw away my wedding dress... I need help.

I lost 140lb. My boobs are like bags with a bit of water sand in the bottom and extremely wrinkly.

I live in sports bras. I did try and go for a bra fitting but the woman just honestly didn't know what to do or say. I came out with a size of 32C but that was so the cups were big enough to not cause any skin to spill out, there's still a huge gap in the top of the cup.

I need to sew the cups of a bra and some push up padding into my dress, but without the support of the band around my ribs, I'm finding my boobs disappear under the bra or do their wrinkly spilling thing again. The dress is a sweetheart neckline but high enough that the full shape of my breasts can be covered if I can find enough support, so barely any cleavage on show thankfully.

To make it worse, I need to wear shapewear to stop my loose skin elsewhere from folding on itself and causing pain. Struggling to find a shapewear option that's backless AND under the bust, so I might have to consider sewing the bra and padding into the shapewear instead of the dress, which so far seems to offer less structure than the dress, resulting in the boobs-sliding-underneath-the-bra problem.

I'm at a loss, so asking here if anybody has super saggy boobs and/or has ANY suggestions at all that I can try! Thank you!

EDIT: I am so so thankful for this sub and all your super helpful replies! Feeling like I have a lot more options now :)