Since it is a custom-designed board, manufacturing errors are quite possible. IO0 at 4V is abnormal; IO0 is expected to have a logic level of 0V (low) or 3.3V (high).When programming, IO0 should be pulled low to enter bootloader mode. An incorrect pull-up resistor value or pull-up connection on IO0 ...

Not all phones implement the HFP battery reporting feature. You can verify by connecting your phone to a commercial HF device that supports HFP (e.g., a car kit) and checking if the battery level is displayed.

You can reduce Frame Size. High resolutions like FRAMESIZE_UXGA (1600x1200) consume significant memory. Lower the frame size to reduce strain:
config.frame_size = FRAMESIZE_SXGA; // or FRAMESIZE_XGA
You can reduce the buffer count if two frame buffers are too demanding:

config.fb_count = 1 ...

Hello, what problem are you facing? Are you seeing any error message?

Found no straightforward demo, but these links may prove to be useful:
https://components.espressif.com/components/espressif/esp-ui/versions/0.2.0/examples/esp_idf/esp_ui_phone_p4_function_ev_board?language=en
https://components.espressif.com/components/espressif/esp_video/versions/0.4.0/examples ...

Since ESP-NOW uses the same underlying Wi-Fi hardware for communication, any Wi-Fi-related activity will block ADC2 operations. Use ADC1 instead of ADC2.

In Development mode, the ESP32 encrypts the flash contents on the fly when writing. In Release mode, you must provide pre-encrypted binaries.

You can set CONFIG_BOOTLOADER_LOG_LEVEL_NONE to minimize logging overhead.

You can switch to a programmable digital servo.

Check if the CONFIG_SECURE_BOOT_KEY_DIGEST or CONFIG_SECURE_BOOT_ALLOW_KEY_ROTATION options are enabled in your bootloader.