I use the ALT HOLD only for the same reasons you do. I chose Expert mode at the time Dynon offered the option just because of the ALT HOLD only was missing from Simplified Controls.
Expert mode is not all that difficult to use considering all the extra options. To take a tour in ALT HOLD mode only, I dial in the altitude bug before takeoff and engage the AP on climb out. The AP takes me to altitude with the current VS and ROLL HOLD. When I'm finished post takeoff checks and ready I disengage ROLL HOLD and take over steering. I can do that while the AP is still on the way to altitude. The AP changes to ALT HOLD when it reaches altitude.
I agree that the ALT HOLD only option would be nice in the Simplified Controls but I will continue to use the Expert mode.
The hack would be to pull the breaker on your roll servo and the AP will change to single axis mode.
