A satin is a better choice than a flat for your exterior trim. I agree that having some sheen will hold up better than no sheen. The satin paint is more flexible than flat and sheds water better.
Will semi-gloss hold up better than satin? I think it will with dark colors. Semi-gloss is even more flexible and a bit more resistant to water and sunlight. Using semi-gloss will take more coats for perfect coverage. This alone could make a difference. (Two coats is always better than one no matter what sheen is used.)
Stick with what you have. If you used a good quality paint and applied two coats it is unlikely that you will have any issues in the near future.