The stand-alone setup is what I prefer. The way I am thinking about trying a gauge is to run a gateway server on an R-Pi that connects to the serial hardware. This is the topic for that:
https://forum.arduino.cc/index.php?topic=571320.0
The idea was to have an R-Pi's serial hardware connected to the micro-controller. Then the gateway runs at a network port (8000) so the webpage can use JS's XMLHttpRequest() to access the WebSocket for updated values. This is where I got to with that project it has some notes that may (or not) help:
I have not used the ESP's, and suspect they have a few ways to do this sort of thing, but I like having the web server port (e.g. 80) separate from the WebSocket port that does stuff to the hardware.