I 100% agree with this. My daughter took Calc BC as a junior and then Multivariable Calculus as a DE class the first semester senior year. Calc BC was WAY harder. Then when she got to college she decided that since it has been a year since she’d taken math and she was an engineering major who would need a lot of math, she retook Multi thinking it would be good to refresh herself and it wouldn’t be too challenging. About 1/2 way through she said she felt like she was taking an entirely different class the material was SO much harder. It ended up being one of her worst grades in her 4 years there. I guess what I am saying is she was very challenged by her AP math classes and not really at all challenged by her DE math class - and the DE class did not at all prepare her for math at a rigorous school. But, as painful as the Multi class was the second time around it did better prepare her for her higher level classes.
OP: Like others have said I would definitely be careful about making assumptions about what classes will transfer, especially to rigorous out-of-state colleges. Schools like Brown (which you mentioned as a dream school) want to give Brown degrees to students who completed the vast majority of their education at Brown. My D had a couple of friends who transferred into her school after freshmen year elsewhere, and they both ended up staying at her school 4 years. I have no doubt the you will be able to get some of your DE coursework to be accepted but in MANY (probably most) cases not 2 years worth. If it was that easy to game the system, everyone would do it.