As a side note, is an optocoupler ran in parallel with the LED's likely to be a plausible solution to this?
The isolation isn't really needed, but you could do that. You'd need a resistor in series with the Opto's LED since it's voltage drop will be lower than the green LED's and it would hog all the current. You'll get pulses on it's output just like the transistor circuit above.
Perhaps that's your issue? Difficulty detecting low duty cycle, short pulses, in software? I don't think that using the ADC on the ESP32/Arduino is a good method for detecting short pulses directly. A logic level input would be more suited for that.
If you wanted to go more old school, you could feed the pulses into a 555 timer (Or a peak detector) set up to give a steady output when pulses are detected. But you shouldn't need to, since you can do this all in code.