Detecting small AC current signal generated by phototransistor
Sunlight can be removed, with a low_pass feedback.

Connect a series R + R network from output back to Vin-.

these new resistors are in parallel with your current_to_voltage Resistor

we need a very slow time constant, so add 1uF capacitor at midpoint of the 2 new series resistors

make both the new resistors of value 1MegOhm.