Enabling and disabling serial device connected to the Arduino HardwareSerial by software
There is no need to add extra active circuitry for this.

The MAX485 has a Receive Enable (/RE) pin.  You can connect this to a GPIO with a pull-up resistor, so that it will be disabled when that pin is unconfigured during bootloader operation.  Once your application program starts up, it can configure the enable pin as an output and drive it low to enable the receive side of the MAX485, which will overpower the series resistor in the ATmega16u2 path.

Since an RS845 bus normally needs explicit transmit enable, you presumably already have the MAX485 transmit enable (DE) wired to a GPIO.  Just keep in mind that unlike /RE, DE is active high, so to make it disabled by default, you'd need a pull-down resistor.