Does anyone have an idea if there is a standalone personal effects insurance so that I can insure my gear whilst in the car/rented van for a WA windsurf road trip (plus the added benefit of other valuables during that trip)?

So far I've only found personal effects insurance paired with a home insurance which I feel I don't need. Does anyone know if there is an insurance provider offering personal effects on its own?

Surely one of the travel insurance providers will have something. Sometimes you can even suspend your health insurance (if you have it) whilst covered by travel insurance when touring.