Conventionally, PID Controls take the temperature as an input and give a proportional 0-10v as an output, which is accepted practice for all BMS.

You then have separate heating/refridgeration controllers which accept the 0-10v proportional signal as an input and control the SSR's.

As separate units they give you many advantages - least of which is versatility. You can have single and three phase SSR's in dozens of sizes from less than 1kW to 100's of kW, yet they'll all accept the common 0-10v proportional input.

Seems a case of reinventing the wheel by welding the tyre to the hub as a single entity...