is there any way to do this without ruining his ratings? i.e. if you DFA him and then assign him to AAA, can he avoid the rating dump, or do you just need to commit to him as long as you want his ratings to stay intact?

yes, do it right after a development cycle.

Doing it right after a development cycle might just mean the system won't create another development cycle event on the player card. The system could still knock the ratings down behind the scenes and you just wouldn't see it until the next development cycle triggers. I can't say definitively how it works one way or the other since I never really tracked it that closely. I just make sure to plan it so I never demote anymore so I don't have to guess how the system handles this aspect of the game.

Can you wait until the playoffs start dean? Once the regular season is over, you can demote ML to minors without negative consequences in my experience (but not minors to a lower minors). You can promote/demote all you want before spring training starts without a ratings hit also.