Have you tried sticky boobs? I buy mine from Target. They don't provide a ton of support, but they are better than nothing. Otherwise, can you get cups sewn into your dress in alterations? That might be a better option. My dress had cups in it so I went without a bra.
I’m nervous about this as well. I’m a 34 DDD and I have a hard time just keeping them in a bra. I haven’t dress shopped yet, but I’m pretty sure I want a backless dress too. Hopefully sewn in cups do the trick. Good luck!
I found these on Amazon. They are not great alone so you will need cups in your dress and they only worked once for me but held up when I performed a wedding on July 4th in Virginia (during the 2019 heat wave) and they stayed up for several hours. They do pull taking them off but otherwise no real complaints.
I'm a 34DD too - i've never been very trusting of built in bras with dresses but the last time i wore a dress with a built in it had a low back with a corset style . My FH corseted me in super tight and i had no bounce issues all night - and we danced for like 3 hours - for added nipple safety though i did wear some pasties. I had originally intended though to wear my sticky bra.
i recommend that with stickies you ensure to check reviews that it will hold your girls in. I was gonna send you the link for the one i used but it's not on amazon anymore - a lot of the ones i've looked at on amazon weren't very good for dd/e cups. So make sure you really check those reviews before buying.
You can also double up if you really need extra safety measures - a friend of mine always recommends to me boob tape and sticky bra together for added security - plus if your dress has built in bra you should be ok.
See if you have a lingerie store in your area (not Victoria’s Secret). I’m also a D cup and had a sheer back dress. You can get a bustier and then have it altered so it doesn’t show. Worked really well.