If you already have an app with no AJAX going on, HTMX is hugely helpful in progressively bringing it in. You just provide endpoints for the forms or other pieces of pages that need updating, and wire them up to the page with the right HTMX tags.
Even starting from scratch, it can be very productive to keep all your HTML in one place- eg Django templates- and use HTMX and a sprinkling of JS to provide client-side interaction. You can jump straight into laying your app out without building the whole thing up as platform-agnostic APIs first. When you need API endpoints you can start adding them based on the context objects you're providing your Django templates, since all the information is in there anyway. There's real benefits to putting all your presentation and most of the logic on the server.
And keep in mind that many web apps will never ever need to also be a native app.
Even starting from scratch, it can be very productive to keep all your HTML in one place- eg Django templates- and use HTMX and a sprinkling of JS to provide client-side interaction. You can jump straight into laying your app out without building the whole thing up as platform-agnostic APIs first. When you need API endpoints you can start adding them based on the context objects you're providing your Django templates, since all the information is in there anyway. There's real benefits to putting all your presentation and most of the logic on the server.
And keep in mind that many web apps will never ever need to also be a native app.