We use bootstrap for layouts. I think you're going to need to write some special tooling if you want to migrate hundreds of forms to the web. Maybe load a form, generate a component tree, dump to json and then try to generate devextreme forms with that information?
That's indeed an interesting idea, will have to do some serious investigation to see if that is possible. If nothing else, it might give us a GUI editor to drive the new forms.
Do you do the form layouts by hand then, or are there other tools to assist with that which work well?