Hi there, first I'm so sorry you have to deal with this.
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Hi there, first I'm so sorry you have to deal with this. It's on a much smaller level but I've always struggled with body dysmorphia and clothes shopping is an absolute nightmare, it doesn't help that sizing can vary so greatly between brands and most of what is carried in stores are for "average sized" people.
Anyway, I really think you'll probably have to just shop online AND probably go to a tailor, based on what you've shared about your shape. It sucks, but then you can get something that fits you perfectly and it will probably feel much better than just getting something way oversized that is super baggy in some spots and not in others.
I found this website called Johnny Bigg that carries dress shirts up to an 8X. According to their size charts, an 8x can fit a 69-71" chest, so I'm thinking that you could get away with buying shirts from there, maybe, and bringing them to get tailored. Here is a nice button down shirt from them. It is possible that you might need to buy completely custom made dress pants though. Do you wear your pants over or under your belly? Not sure your exact shape but if you are able to wear pants under it, that might not be preferable for you but can cut down on the size significantly.
