Ruggedizing a home-made High-Current SSPC
Your Specs: (my summary)

Input


Input from 3.3V or 5V uC output.    
input reverse voltage protection.    
1,8~24V digital input ( if possible)


Output


External 12V supply to series low-side switch , galvanic isolated from input.
Output switch RdsOn ~0.0012ohm


IR opto LED characteristics



All LEDs can be simplified by \$V_f = V_{th}+I_f*ESR      \$
From the (typ) graph, \$V_{th}=1.36V~ @T_c=25'C ~~  ESR = 2.86Ω~ \$(= 0.1V/0.035A)
However there is a large tolerance on this as indicated by VOM1271 datasheet table LED forward voltage \$V_f= 1.2V_{min}, ~1.4V_{typ}, ~1.6V_{max} @I_f = 10 mA ~~\$

Comments on proposed design in question


It is the wide tolerance on ESR that affects Vf vs If as well as Temperature shift in threshold voltage Vth,  that makes using a fixed series R not suitable for a wide input voltage range when a limited current range is desired.   
An input 3.3V to 5V is possible but one must analyze the ESR of the logic voltage and include this in the voltage drop using Vol/Iol.   
for reverse voltage protection rather than shunt diodes, you use a series Shottky diode or seriously prevent reverse voltage by design.

Recommendation:


Review example specs for Automotive Smart low Side switches, there are lots of these with better performance.
Then revise your design specs as req'd and start over.
eg. 

https://www.digikey.com/product-detail/en/infineon-technologies/BTS500101TADATMA1/BTS500101TADATMA1TR-ND/6565755     

https://www.digikey.com/en/product-highlight/i/infineon/high-and-low-side-switches