@hgptamn
The method you need will depend on the implementation of the ESP32.
So my questions are:
1) Would you use an existing module (WROOM, WROVER, ALBWROVER etc) in your project? In this case I guess you need programming and verification before soldering
2) Do you design a circuit yourself, with the ESP32 mounted on your board? If this is the case, than the question here is why do you want to disable the ESP32 while programming? (do you want to avoid powering up other components on your board?)
3) You want to be speedy with the programming (that's understood in a production environment), than directly interfacing the flash is a way to go. I think there are programs already available for interfacing an external SPI-flash, for which simply writing to the programmed flash should not pose any issue.
If you only want to program an existing module, there are jigs you can buy online where you snap the module in, program it with UART, remove it from the jig and solder onto the board.
Check my post below. This module first WROOM and ALBWROVER modules. There is also a version for the WROVER module (I currently have the WROVER verysion).
https://esp32.com/viewtopic.php?f=2&t=4180
Another way could be to have the UART pins exposed as test pads, as well as the 3.3V supply, and use these points for interfacing the ESP32 for programming. For this a test fixture can be used with ease (contact using POGO pins), or a programming connector, can be used. Using this method, programming and functional testing afterwards can be done within 1 fixture - a good solution for production environment.
If you don't wan't other stuff on your board to come live while programming, you can use isolate the power line that is only used for programming, using a MOSFET (There are MOSFETS that are so small, they are as small as a resistor, so not really consuming much space) This way - since the programming is managed on the ESP on bootloader level, and this should not interfere with anything connected to it - then you can use the UART for programming. And you can of course use another ESP as ISP in this case but using UART. If you strap the pins right on the target ESP, than it will wirte to flash the data it received on UART from the programmer ESP (as fat as I know).
By the way, interfacing only the flash in my opininon looses some options for configuring the ESP, and it means loosing some features such as flash encryption or secure boot, if this is going to be a commecrial product.
