Open Drain Serial TX output on Arduino
This is messed up a number of ways:





You say the digital output is open drain, but you have provided nothing that will make the signal go high, ever.

A 60 V 55 A MOSFET is gross overkill here.  At least this one can be turned on usefully with only 5 V gate drive.  So while silly, it should at least work.

The transistor inverts.  That means the transmitting and receiving UARTs have to be configured for opposite polarity.  That is not possible with most microcontrollers.  Such UARTs are generally fixed with the line idle level being high.

You mentioned nothing about baud rate or hardware protocol, like 8-N-1.  This is not something you can overlook.

The baud rate has to be slow enough so that the passive pullup can pull the line high in a small fraction of the bit time.

It's a real head scratcher why you didn't just connect the two pins together directly with a pullup.  How is that not the obvious thing to do?  There is no need for a hairbrain scheme here.