I checked the motor calibration and they all started together so maybe they were calibrated and I don’t remember. However, you are correct about the C5 and C6 throttle being much higher than C7 and C8.
It looks like I have QUAD and H-FRAME set up so my assumption that C5 and C6 throttle being on the front two motors was incorrect. The motors for X or H frame are in the same position but the rotation is different. If there was a CG issue, I would expect C5 and C7 outputs to be saturated or C6 and C8. I can’t explain why C5 and C6 are saturated so perhaps I need a re-calibration to eliminate this as a reason.
Channel 5: Front right motor, clockwise
Channel 6: Rear left motor, clockwise
Channel 7: Front left motor, counter-clockwise
Channel 8: Rear right motor, counter-clockwise