Humidity spiking issues

Hi guys just a quick one. How can I stop my humidity spiking when the lights go off and prevent mold in the late flowering stage? Would another dehumidifier do the trick? Do I need more fans than I've already got? Or do I need an oil filled radiator for when the lights go off? Any help is appreciated

You need more dehumidification.

Raising the temp with a heater will help to a lesser degree (it's the relative part of RH).

The humidity is spiking because the temperature is suddenly dropping when lights go off.

I would start with increasing your exhaust speed, assuming the area you're pulling air in from is at an acceptable humidity level. Faster air exchange in/out of the space will suppress rapid changes in temps and humidity.