How about copying a proven design, e.g. [reprap.org] ?
To have it on by default, simply connect the 1M resistor not to GND, but to 5V. Using a TIP120 instead of an IRLB8743 is entirely fine if the load is below a few amps. And ... TIP120's are slow, so no 20 kHz PWM. 50 Hz likely works (FAST_PWM undefined in Teacup firmware).