The esp32-hal-ledc.h file has the following prototype:
double ledcSetup(uint8_t channel, double freq, uint8_t resolution_bits);
You could try and just set the frequency to 0.5. e.g.
double PWM_FREQUENCY = 0.5;
You can find the file under
C:\Users\UserName\AppData\Local\Arduino15\packages\esp32\hardware\esp32\1.0.4\cores\esp32
There is a lower limit to how low the hardware can divide the clock. You can find more information in the ESP documentation. The ESP-IDF API is different to the ESP Arduino API but the hardware information is valid.
https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/peripherals/ledc.html