Choosing a carrier device without being on that carrier for the rest of your devices would seem to be the first mistake. Treating the carrier as anything other than dumb pipe seems like the issue here. Going with the Pixel Watch LTE and then doing same custom app development might make more sense for the described use case, but I haven't explored the author's use case thoroughly.

Good point, consumers should all simply develop their own apps instead of buying a product.

How would the custom app disable the native Watch services?

So you blame the victim and also admit you haven't explored the author's use case. Very helpful