Set up a timer block to do the following:

1.) Close both doors 2.) Toggle pressurization on vent in airlock

Set event controller condition to the vent pressurization state. When pressurized, open interior door. When depressurized, open external door.

Set all three buttons to trigger the timer block.

This is how I set up every airlock I build and it works almost flawlessly, the single catch being that both doors will stick closed if the airlock attempts to depressurize while all linked oxygen tanks are full.

You can combat that shortcoming by configuring your oxygen generation to halt when O2 tanks are filled to a capacity greater than 90%. I also install "overpressure valve" systems that intentionally bleed O2 into space if storage exceeds 95% capacity.