So, both players' frequencies make a big difference here.

The player raising from MP is playing 26/8 over 132 hands. His raise first in from MP is 5% so far, 1 out of 20 opportunities. Given his tightness, my plan was to call with my premium rundown when he raises. I'm not sure his actual MP raising range is 5%, because that's a bit ridiculous...but it can't be much higher, given it's 8% from all positions over a non-tiny sample.

CO is a donk playing 75/36 with a 25% 3bet over 37 hands. When he 3bets, I feel like all my options have merits here.

My main concern with flatting is that MP might decide to 4bet his KK hands facing a 3bet from a maniacal, short-stacked SB, and a call from me which signals I don't have AA. If he 4bets AA/KK, it's quite easy for a tight range like his to reach a high 4bet frequency here, which is deadly for me. I stand to lose my 9bb every time this pot gets 4bet by MP, so the profitability of my call is very sensitive to his 4bet frequency

4betting is an interesting option if MP will 5bet only with AA. We might not even have to go with a pot size 4bet here. We can go smaller and fold to a 5bet. I thought about this option in game, but decided to go with the safer route.