Unless I misunderstand, the last two computers I use to look at the Davis output I have read into the Weather Display program using a very reliable USB to serial device. I also have a couple of RS-485 output devices talking to my device that receives that data via a DB9 and converts it into a USB to gather the data.
None of my computers for several 'generations' (old ones fast enough to do the job, finally fail, get another one that is newer but still old) have had a serial port that I've used.
Is this a possible solution, but I'd have thought you'd have tried this work around by now. Dale