question

Thanks! Where can I learn more about adding an assistant in software?

You could also choose a larger or more powerful Multiplus II, say a 48/8000 or 48/10,000 or even a 48/15,000 (but this last would likely be overkill for your project). Alternatively, you can install 2 x 48/5000's in Parallel for an equivalent of a 48/10000.

You would also be able to use an assistance to disable that water heater if required.

A couple of points...

5000VA is not 5kW unless th load is entirly resistive.
I cant remember the spec off the top of my head but I think a 5000/48 will be rated at 4000W continuous at 25C.

That asside... The AC2 output can be used, to do what you wat from a switching perspective, I think, The strategy is gennerally refered to as load shedding.

I think you can use multiple rules, on and off, to achieve this in the relay assistant but a simple single setpoint will not work because turning off the heater would reduce the load causing it to turn back on.

Avoiding this common issue requires hysteresis in the control logic.
EG. Turn AC2 off above 3.5kW and turn AC2 On below 1.5kW

You may also want to add disabling AC2 blow some SOC value.

The relay assistant accepts multiple rules and processes them in order. You can choose if processing stops when a rule matches some criteria.

The order is usually importent. In the example above the SOC check would come first and prevent load checks from taking place if it matched.
You would also want some hysteresis for that so entry 2 would enable above some slightly higher SOC.

This video is the opersit of what you want but the principles are the same.

You could also do what you want and much more with Large OS and Node Red on a GX device (Not a CCGX though)

hope that helps.