I've been making a similar device for the past couple years for my own use. See picture.

My method is to to do a Receiver Valid test at power on. The pulse from the Rx has to fall in the range of Gear Down or else it waits. In the meantime, no pulses are sent to the servos. I save a Gear Status word to EEPROM which represents the last valid gear command and is written when the gear cycle is complete.

So, when a valid Gear Down signal is received, it checks the Gear Status word. If the gear is down, nothing changes and the sequencing code is not executed. If the gear is up, then it runs the full gear down sequence. It doesn't matter if the air pressure is low and the system is in Failsafe. It will still try to put the gear down.

I can turn on, start up and try to taxi out with the Gear Switch in the up position - except steering and brakes won't work - and then remember to put the Gear Switch down and continue on. I've done that more that once and it's saved me.

I also included a button which you can use during assembly and tear down to cycle the gear without the transmitter. It overrides Failsafe and will operate the gear regardless of air pressure. There is a 1 second hold down period before it operates. It also can override a Failsafe situation and let you operate the gear with the transmitter (regardless of air pressure). The Button has proven extremely useful, day to day, and for testing.

There are two kinds of receivers - those which send no servo signals until link up with the transmitter and those that always send out something (Spektrum). To use the Button this way you have to account for the different types of receivers or whether a receiver itself is in Failsafe (Tx on then off). So I have two different delay settings on the Button to provide the right action.

I also include a proportional brake channel (gear down) and, optionally, a proportional smoke channel (gear up) via transmitter mixes into the gear channel. The resolution isn't all that great but do you really need more than 200 or so steps for brakes and smoke? Gain is two Tx channels at the expense of 2 mixes.

Sequencer is programmable using a plug in dumb LCD terminal with navigation switches and button.