I have this TTL UART to RS-485 board: https://www.aliexpress.com/item/5PCS-MAX485-RS-485-Module-TTL-to-RS-485-module-Raspberry-pi/32579369267.html
On Omega2 at least at the RO pin of MAX485 some kind of 5V to 3.3V level shifting is needed.
The simplest solution: put a voltage divider between RO and RX1.
And you should connect the pull-up resistors of input pins (DI, DE, RE) to 3.3V.
RS-485 can be made full-duplex by using four wires ie. two twisted pairs.
Is your Modbus slave full-duplex?
BTW you could build also full duplex config with two of your MAX485 modules.