After a fair bit of troubleshooting, I now have a major part of this project complete:

The ESP32 can now be programmed remotely from a web interface

I'm still working on the UI, but for now it's okay:

At the top is a code editor, with full syntax highlighting. Shown is a simple example of a capacitive touch sensor program for the ESP32

The general idea is that each function in this file can be called remotely as a part of a larger routine initiated by the Google Assistant. For example, there could be a function turn_on_tv(), which uses an IR LED to turn on a TV. When the assistant recognises the phrase 'turn on my TV', it calls this function and the TV is turned on.

Once code has been written, the Upload button triggers the compilation of code to include a lightweight web server, connects to the ESP32 via WebREPL and transfers all the required files.

To test the code, the page also has a testing area where the user can pass in any values that the function requires, and get an output from the ESP32:

There are a few UI changes that need making to tidy things up a bit, and at the moment it struggles a bit knowing they type of any values a user gives in the test area, but I'm very happy with the progress that's being made