First, overthinking this stuff is a good thing IMO. If you're like me, you really want to do all the right things to keep your boat in great shape and working like it's supposed to. I can tell you the first 1-2 years of ownership is full of these kinds of decisions, and then after that you just do it all automatically and don't worry about it anymore (or you stop doing it because you don't feel like screwing with it anymore, but at least you know what you've decided to stop doing and why

).
My opinion is that if you boat most of the time in fresh water, use magnesium anodes. The occasional use in salt shouldn't hurt anything, but it might be worth investigating whether ACPS would help cover a little better during those times. The
last thing I would do would be to keep the aluminums on there.
Brackish water is water that's more saline than fresh, but less saline than seawater.