Would it be possible to use an Arduino as a sketch uploader only using the UART interface?
I have an Arduino Leonardo (CJ-MCU Beetle version) and I'd like to use it to upload sketches to my Pro Minis by connecting the Leonardo to a PC with Arduino IDE on it.
I'd like to avoid the use of SPI and program the Pro Minis only by using the TX/RX pins.
Would it be possible somehow?
What you are asking, is whether you can use the Leonardo as a simple USB to Serial ("TTL") adapter, given that your Pro Minis have the bootloader programmed. ISP is in no way involved.
The problem is in respect of the auto-reset function and that is not easy to get around with a Leonardo. Better to simply go and buy a dedicated USB to TTL adapter which includes the DTR function using a CP2102 or CH340. Dirt cheap, the only problem being how long it takes to get them from China.