Hello @brystmar! To be brutally honest, I am not a python or C programmer by any means. I have been using google, and just searching and patching lines of code together, and think I did ok to get here. I am confident there is a more elegant solution for all of this. 
So - certainly if you know of a better/different way - I am all ears!
As it relates to the 8266 and doing this - there was a reason I used the ESP32, and I think it had to do with the number of UART’s on it, but I don’t specifically recall what I read that pointed me in that direction.
To answer your question regarding the uart_readline_sensor.h:
Not knowing how to program in C, or utilize the lambdas well enough to get the information out of the ESP32 that I have connected to the Navien, I utilized the Custom UART Text sensor listed here. Then, I utilized pieces of the code found in this thread. And coupled it with information found in this thread to get as far as I have. Again, Certainly not elegant, or likely efficient - but none-the-less, a semi-working copy!
You are more than welcome to refactor, or help with further development on this as I am sure we could all enjoy not having to use the Navilink Solution. Please let me know if I can provide more information.