But the evidence is mostly just “they wear a shirt to the pool” or “don’t like things of the opposite gender role”

And in most cases, it’s more of a self esteem issue rather than trans. Dipper is a nerd and being a nerd, it’s much more likely he was bullied for the most trivial things like his appearance or the fact that he’s weak. He also like icelandic pop singers from the 80s. His self esteem is was shattered before the show started and is’s very visible in the show. Like how he always tried to be better than mabel at things. He isnt’ trans, he’s just a nerd with self esteem issues.

Danny is bullied for being weak and the fact that his parents are ghost hunters which people considered a joke profession until the ghostly activities happened in his town. He too has had his self esteem broken before and during the show and his parents cause anxiety problems considering they will kill him if they knew what he was. While the “hide who you are from everyone” is a reference to the LGBTA+ community, it does NOT mean he’s trans.

Lars’s self esteem problems are the worst though to the point where he fakes who he is every minute and tries to please those he considers worth it even the most ridiculous request. He will put down others to make himself feel better and try to be accepted for something he’s not. He isn’t trans, he’s just has self esteem issues.

TL:DR the characters do those things because of self esteem problems. They aren’t trans.