You have a PTAT voltage across R1.
VPTAT = (KT/q)ln(m)
This would fix the current in the PMOS current mirrors as a PTAT current,
IPTAT = VPTAT/R1
Your Output Voltage Vref = (IPTATR2) + VBE,q3
Now adjust R2 to get Vref as constant across temperature as possible.
You might require a few iterations to get to the final value.
If R2 becomes too large, try increasing the PTAT current by reducing R1.
One easy technique is to plot the slopes of the CTAT voltage and the -1*m*PTAT Voltage over each other and find the value of the multiplier m for which the slopes match. m = R2/R1