Shirts are one of those things that tend to look bad if they’re not well fitted. I don’t have much experience tailoring for people who are binding, but my experience making and modifying clothing for myself and others has led me to be one of those people who evangelise the benefits of getting high quality staple clothing tailored (though finding a good, reliable local tailor can be difficult for some).

The standard advice for shirts is to buy shirts that fit the shoulders, and then get them tailored. I don’t know if that advice would still apply for people who are binding (I know that people with breasts often need to size up to allow enough space in the chest, and that may remain true when binding), but I would assume this would still apply.

You also mention fit issues around the butt. A transmasc friend reported that he had best success finding women’s trousers that were in a more masculine style. He reported that although he struggled with gender dysphoria due to knowing they were women’s trousers, how the clothing made him look caused to feel gender euphoria.