I'm very confused about what are functions of long pressing either C1 or C2 buttons. Currently if I long press either the DJI GO app seems to reset the connection with the remote. If I hold longer remote starts beeping. Cant seem to find a way to reprogram the long-press functions anywhere, either on Apple or Android devices. It used to be possible rto adjust the gimbal horizon position and lens focus a while ago but not anymore. I've searched a little for info on this but couldnt find anything that'd make sense or explain most current state of things with latest firmware.
Below are currently installed firmware versions on all hardware:

App v3.0.1Aircraft v1.6.1.30
RC v1.7.40
X5R camera v1.9.1.10+

The gimbal horizon correction is done via the C2 + ISO wheel. The long presses that you're referring to are generally used with button combinations that help to either manually reset the RC or link the aircraft when needed. Those functions cannot be reprogrammed. You only have control of the short C1 or C2 presses via the RC section of the app. Could you happen to verify your aircraft firmware again? It seems a bit off.