Why does my mcu keep dying?
I agree with Bimpelrekkie's comment.

When the MOSFET is turned off, GND for the LEDs floats, but they are still connected to +5V. As the ESD diodes inside the LED controller chips conduct, the inputs LED_MAIN and LED_BACKUP are no longer inputs. Instead they output a voltage close to +5V, which destroys your ESP32. Besides, if the ESP32 outputs 0V, it will act as an extra ground connection for the LED chips, so they will power up somewhat through that. Enough current can flow to blow both the ESP32 and the LED controller chips.
It is always a bad idea to switch the ground of a load which communicates with a micro via other data lines. If you want to be able to turn off the LED strips completely to save power, you must do so with a high side switch in the VDD line.