Anyone using CI with React,Vue,Svelte or Marko?

I did a couple of tries with VueJs -> nothing spectacular, it just works.

Here is good advice on Ajax calls setup for reactive frontend frameworks:
https://codeigniter4.github.io/userguide.../ajax.html

And when you will be building your backend, you should ensure that you follow some basic architectural principles and constrains. For example, you should split all communication between FE and BE into small requests, serve each piece as JSON from dedicated controller/method, and gracefully process errors. Those pages in user guide will help you to start:
https://codeigniter4.github.io/userguide...ponse.html
https://codeigniter4.github.io/userguide...onses.html

Then basically all what you have to do if you are building VueJs SPA is to serve your layout that initializes VueJs, and let javascript do the rest. CI4 will not be envolved more then responding to Ajax requests coming to API methods you defined.