Surely there is something wrong with that "on-off speed controller". The way you describe it, is not normal, no motor speed controller should do that. Contact the seller and request a repair or refund. Is the saw OK if you run at full speed?

There is also no way of knowing that the dimmer you are showing is for inductive or resistive loads, and since it seems to be a cheap eBay version, I'd suspect it is NOT as you describe it, it can definitely not handle 4kW inductive load.

You say it is for a mini table saw, but how is the motor? What sort of brand and design is that? What are the motor specifications? I have a Proxxon mini saw with 85W motor, I don't even dream about reducing the rpm, because it would just stop. Also my large Bosch is always running at full speed, even though it has a 1.8kW motor. Why would you want to reduce the rpm? You know that by doing that, you also severely reduce the torque.