I agree, I would never put a sump pump on a small inverter. The best rule of thumb is to double your start-up power (IMO). I have a well pump that I run at my cabin that has one heck of a start up draw - fortunately, I don’t use it as a pressurized system (I use it to fill a tank in the beams of my cabin and allow gravity to pressurize my lines).
We built a gadget that would handle the startup (duplicating the soft-start circuit, but we didn't know that) and decided, to heck with it, go for a bigger hammer.
/johnny