[Fixed]-Django forms with ajax, what would be the best approach?

What you explained is much more related to your client (browser) and just a little to your backend django code running on the server. The backend can handle your data and it can also render pages for you, but if you want fancy dynamic stuff running in the browser you need tools that work there.

There are plenty of frameworks available e.g. vue.js, react or angularjs to name a few.