Watch out on the Pi I believe the GPIO are not 5v tolerant ( 3.3v only). Also the current drive is probably not enough for the relay coil. A simple N-FET of suitable size and a resistor would solve this. 10k from GPIO to the FET gate and the FET on Source on the ground side Drain to the relay. Diode across the relay for EMF if the relay does not have one internally.

I moved from X10 to Hubitat. I am not sure but I think Hubitat supports shelly directly. Might be an option. You could even replicate the photocell with one of the sensors that includes an ambient light level sensor. Many of the motion sensors have this feature.A couple of rules and you would have it.

Did the photo cell get flakey or is it possible there may be some new device causing interference?

If the unit is bad you could open it up and see if you might find a bad component, maybe an old leaky capacitor. I have a pile of old x10 parts but sorry, no photocell units.