>you have to start from the beginning and write an API now.

You could refactor your code to share mostly everything . I have no idea how this templates system work though so maybe you need a different one.

so the API returns an array of users object as a json the html side of things would have a render function that has as input the same array of user objects. What you will probably lose versus complex frameworks is magic stuff like 2 way data bindings but from my experience those always caused issue, I prefered the old react way of doing things, it was more "functional style" and less magic under the hood and I could(and used) JS and not a special template language with special keywords to render stuff.

Amway my point is yu can reuse the API code for the html render stuff too.