How do I disable use of IMU in altitude calculation?

I would like to utilize only the barometers and GPS in the calculation of altitude by EKF3, owing to apparently excessive sensitivity to (or perhaps aliasing of) vibrational noise in the IMUs. I realize I could set EK3_ALT_M_NSE to a small number, but this would also de-weight the GPS, which is not my intent.

My reason for wanting to try this is that logs show correct baro pressure and GPS altitude, but the HUD display of altitude in Mission Planner was wildly wrong, always fluctuating within +/-20 m of 0, despite multiple ascents to over 100 m.