How is the MOSFET temperature with the new resistors?
For your MOSFET power calculations, I'd say 2^2 rather than 4. It is clearer, then, that the number is 2A squared.
Varistor Voltage (Min) 19.8VVaristor Voltage (Typ) 22VVaristor Voltage (Max) 24.2VCurrent - Surge 500A
Maximum DC Volts 16VDC
back to back zener diodes
Would a lower value for R2 help here? Any suggested values?
On a side note: 1N4001 is a bit slow for this application. I usually see 1N4148
I would also use a 1N4001 diode, that one is just a signal diode.