100 blinds
Lag vs Lag (I’ve 3! him a lot and he opens a lot)

Btn opens to 6
Sb calls

I raise 77 to 25.
Only btn calls

What flops should I cbet and how much?
I’m thinking half pot all non-monotone boards. Is that a good plan?

Should I cbet A high boards or boards with two or three overs?

Plan is too generic. If you are going to 3-bet like this you need to c-bet a lot but all non-monotone would be too many. Plus you have the problem here if only been $175 effective and having a roughly $50 pot on the flop. With SPR < 4 you need to ramp back your c-bets a bit and be ready to carry through with a shove on the turn when you do c-bet.

77 is a good hand for calling the $6 but it isn't a good 3 betting hand. For $6 you can mostly set mine. Once you make it $25 your bluffing with a hand that had value to begin with. With stacks in that range your 3-bets should be hands with high cards you can commit to when you hit and low garbage you can c-bet and give up unless the flop is particularly favorable.