What you are effectively seeking is the Firebase Admin SDK as opposed to the JS Client SDK. I have managed to implement this on my server and using it with Node JS and am looking into it for my PHP development. After communication with ‘Firebase support’ and a thread in the github community, I found out that unlike the Client SDK the admin sdk is not available on the web client side. As you say, this is not a Thunkable issue but a Google design parameter.
If you do find a workable solution, please update this thread and of course I will do the same.