My objective is to deploy new versions of firmware on chip Atmega 8 using UART port (RS485). I flashed a blinked led with bootloader (using MightyCore bootloader built with Arduino studio). I have burned it with other software on programmer board.

Now I have connected the computer to board using COM serial port (UART of chip with RS 485), but when try to flash a new version of firmware (arduino studio/avrdude), it shows the message that it can’t sync with board.
Could you help me to understand what steps I’m doing incorrectly?

My final objective is be able to update the firmware of multiple boards on RS485, forcing to other board disconnect the UART port while one is updating. If there are other way to get the objective, tell me please. I thought to build my own protocol to send the new firmware but I think it is more risk and difficult to get the objective correctly or not?