The band size is the best place to start when determining bra size because that's where your support should be coming from, a snug band that isn't going to move around, especially on a strapless style!
You can use the grid in the Bra Fit Guide to double check your band size, take the measurement under your bust around your rib cage; 25-27" is a 30 band and 27-29" is a 32 band. If you've been wearing 32 DD, the equivalent cup size in a smaller band is 30 DDD. I would definitely go to the 30 band if you're wearing your 32s on the tightest hook. A brand new bra should fit snugly on the loosest hooks so that as it stretches with wear you can adjust to a snug fit for maximum support.
This particular Le Mystere style has a little push up for lift and support and is best for a fuller profile. I've linked another strapless style below that comes with removable straps by Simone Perele that is available in a 30 F(DDD) and is best for an average profile.
Take a look at the Bra Fit Guide, scroll to the bottom and you can see definition of profile types: Shallow, Average or Full. If you do have one breast that is slightly larger or fuller than the other (totally common btw) you should always fit to the larger side, similar to shoes, you fit to the larger foot. The guide and bra fit video have additional tips that should help you understand some of the terminology surrounding correct bra fit.
Once you understand your profile fit, you can better order styles online. If you find the 32 DD or 30 DDD is still too small in the cup, particularly on one side, try going up to a 30 4D. Sizing does get a little tricky by company. Chantelle, Le Mystere, Wacoal and Simone Perele F=DDD and G=4D. For Fantasie, Freya and Panache E=DDD and F=4D.
Essentially my advice is to try the Le Mystere or Simone Perele style in 30F(DDD) and if those feel too small in the cup, try the Fantasie strapless linked below in a 30F(4D). Of course, stopping by a Nordstrom store to get fit is always a great idea!
Hope this helps!