Just to get it right.
You set CH422G’s IO2 as a switch with default state to ON to turn ON the backlight. Same as “stock config”.
Then with GPIO16 from ESP32-S3, you create a LEDC output component, wire from RS-485’s “A” pin to the testpoint labeled above.
With correct ESPHome config. You just leave CH422G IO2 always ON and just use your monochromatic light entity to control LCD Backlight brightness.
The backlight control of CH422G IO2 now just becomes a static output always set to ON. The actual setting of backlight On/Off and brightness is now completely handled through the monochromatic light entity mapped to GPIO16.