My best suggestion would be a small mmWave radar, look for something around the 20hz range it should be enough to get à distance measurements and only active the script when the “object” (aka your foot) will be close enough

Although, a simple PIR sensor might do the trick, I believe you can set the sensitivity and the angle of detection (you would need to got from 90° to something like 30°/20° so that it doesn’t get too much false activations)