If a biological female who is already feminine takes a LOT of estrogen regularly will this be dangerous and why?

The body can only effectively use about so much estrogen anyway, so with that she'd probably be at the maximum limit constantly which isn't something you want. Large numbers of Estrogen tablets also come with a high risk of life threatening blood clots. So taking that many would definitely be dangerous even in the short term.

Then as was previously said estrogen fueled cancer. There would be mood swings and all kinds of other shit too. Basically there's no reason to be taking estrogen like that unless you need it because of health reasons.