> Does this mean you boot the web app in a native WebView in the background (not visible), while showing a [splash?] screen until the web-app sends a signal from the web-view to java (or swift?) that it's ready?

Yeah, that's what the RN codebase did, and for our initial switch over to native, we did the same thing there too.

These days we launch into a native "Home tab" view that lists all the pages in the workspace, as well as recently edited pages. Our philosophy is to progressively convert views to native, working from the outside in towards the editor. Soon we'll be launching a beta for native-ifying another of our top-level tabs.