I was able to finally get the project to build for the right board by following your example.
The solution looks like it involves these 2 key steps:
-adding a “boards” folder under the project root directory (C:\Users\me\Documents\PlatformIO\Projects\nixie_watch) containing the “nixie_watch.json” modified as we discussed.
-adding a “boards” folder under C:\Users\me\Documents\PlatformIO\Projects\nixie_watch\zephyr. The project then utilized the “nixie_watch.dts” devicetree under C:\Users\me\Documents\PlatformIO\Projects\nixie_watch\zephyr\boards\arm\nixie_watch as opposed to the .dts file I had originally placed in the C:\Users\me.platformio\packages\framework-zephyr
That tutorial you posted looks great. At least I can begin the long grind that will be the first rev of working firmware!