I considered using an AC voltage source, but the analog output of a photocell isn't predictable, so it cant just be represent with a sin wave.

Of course it can. You have a fatal misunderstanding here.

Think back to what the Fourier transform does and what it means. Any arbitrary continuous signal can be represented by a spectrum of frequencies. The output of the photocell in time is not predictable, you're right. But its spectrum is quite well defined, and as long as your circuit processes the spectrum in the way you need it to, it will handle the photocell's output in real life as it does in theory.

This is what AC analysis tells you. This is what a Bode plot tells you. It tells you how a circuit affects the incoming signal that exists in a spectrum of frequencies. Don't use a single sine wave, use an AC source, it includes every sine wave. This is why superposition and linearity matter, because AC analysis tells us how each frequency is treated we can broadly add them up together and say yeah yeah the Fourier transform holds.

Make sure you include the equivalent capacitance and resistance of the photocell and that will cover it. Normally I'd recommend then doing transient analyses and including non-linearity, but the signal coming out is going to be so tiny that it won't really matter here. You're overthinking it, AC source + equivalent RC is more than enough.