I wish you the best on your Kickstarter project. If I may suggest something about Kickstarter the video. IMHO, the video would have been much better if you had shown it in action.

Actually you dont need to do it, its enough to pull down IO0 and then connect to power supply. Sure, you can do it that way as well. But why bother when the EN and IO0 signals are built into the programming adapter. Just run the extra signal line. If your going to have a way to pull IO0 low, you mi...

You do not need a "button" but when flashing you need a way to pull EN low. His schematic has no provision for doing that.

An interesting thing is, when I hold the EN button on the DevKit and run the command above,

Not really. Holding the EN down hold it into a reset state.
how about providing your JTAG pinot to the module.

Just curious why your using Serial1.end() ?

Disables serial communication, allowing the RX and TX pins to be used for general input and output. To re-enable serial communication, call Serial.begin().

https://www.arduino.cc/reference/en/lan ... erial/end/

Then I push the "Boot" button and "EN" button same time and release the EN button.

The correct way is push "Boot" and "EN" buttons at same time then release "EN" button, then wait about 0.25 sec and release "Boot" button.

Have you had a look at the I2C examples?

https://github.com/espressif/esp-idf/tr ... herals/i2c

Can i use http or tcp for file transfer while having the esp32 as an AP?

http uses tcp (mainly).
tcp is to ensure your data get there correctly with no dropped packets. udp is a send and forget method. udp is faster, but you are not assured that all the data will get to the client.

Why did you post your question in the "Showcase" section of the forum.

This is a ESP32 forum.