You need to be able to switch 300A? The benefit of the SSR is to be able to cost effectively switch the battery power to the ESC on/off via a low power switch. Would also be useful for emergency kill circuit. Generally SSR’s greater than 100A are very expensive. If these are accurate specs then they are very good value. Note that depending on the quality of the SSR they have a small voltage drop and will generate some heat when conducting, especially at very high currents.