Gosh. I shouldn't be surprised by S&B threads any more, really - but some of these suggestions are quite depressing ...
OP - I'm about the same height & size as you, also large of bust, & four years older. I appreciate that the conventional requirements of a military function will restrict your choices a little - as will keeping costs as low as possible. But please - there's no need to resort to Roman, JJ's House or, god forbid, Shein or Temu!
Of all the suggestions, the one that looks best is the teal Ghost one. It's simple & striking & doesn't look like a danger next to candles (most of the others look as if they might combust spontaneously ....!). Ghost viscose satin & crèpe are the brand's signature fabrics & are good-quality semi-synthetics with weight & fluidity. That dress mightn't work, bust-wise - but it's worth looking at some others.
Charity shops & Vinted/eBay etc, great in theory, but you'll need to be extremely lucky to find the right dress in the former, & the latter options are, as you say OP, a bit risky in this instance.
However, buying from Vinted/eBay will offer you the opportunity to find something beautiful, which was originally much more costly than any of the suggestions above, maybe even in silk or another high-quality fabric. I would suggest checking out certain brands in the flesh to see if they are generally long enough, generously cut in the bust etc. Some brands - eg Ghost, Rixo (which appear on secondhand platforms frequently) do new versions of the same dress every year - so if you try some on in-store/by ordering online, it will allow you to be much more targeted in your search, & minimise the risk factor.
So that would be my own preference in your situation, OP. Does it have to be a dress btw - or could it be a matching top & wide trousers in silk/satin/velvet?