Drop the dogma, know what you want, have a plan before each step starts, and work incrementally. Agile doesn’t account for the reality of programming for corporations, although I think it can still be useful in consulting contexts…. Maybe

It would probably be about the same. Most of the work is in the API functions that the web and mobile are both going to be using. Serving the web page is a pretty small load on the server side, even if it runs a JS frontend.