Any particular reason why you limited your highest baud rate to 300?
Does it have to do with the power transfer, distance or just you did not need higher baud?
What kind of current can you safely deliver to the slave? I need 50 – 100 mA.
I can only guess that you used a diode on each data line and still power from them. If so what is the ratio between signal / no signal timing in your system?
I need a 20 ms frame within which 10 to 12 bytes will be exchanged between the master and slave with a minimum 5 ms free, on each side, for other tasks. This forces me to use 19200 or higher baud rate.
Can you share a concept schematic of your setup?

Thanks again and again.

Nick