I tried flaperons twice and found, like you, that they don't work well.

I think your flap idea will work fine. You're going to inset them into the wing in front of the ailerons I gather? There may be some trim change requireing some down elevator but I think you'll find that it's minimal and reduces to very little if any being needed once the speed lowers. Split flaps generate more drag than lift so the pitch change should be minimal.