I haven’t tried CodeRed yet, but it looks interesting. Nest is DOA after Google shut down the API… I replaced mine with an Ecobee and will never touch another Google product. I had to rebuild my Homekit one time also, and as painful as that was, the system seems to have matured and strikes me as very stable lately. With a little work you can take pretty much any switch or script in HA and have it show up in your Home app (so that Siri also recognizes it).
As for your Airplay aspirations, you inspired me to write a followup to my first post… I struggled with this and finally figured it out. See here for some inspiration… I hear your frustration but stick with it and you’ll be glad that you did!