Can I get the full-control of an cross-origin iframe by proxy the iframe on my server?

Here I need to implant an iframe in my site, and I want to inject some script or css to change the iframe, because of the iframe is not same-origin with my site, so I set a proxy makes it same-origin(localhost:4000).

When I try to get the element from the iframe, an error occured in the console which doesn't meet my expectation.

My site was hosted on localhost:4000 too.

Any idea about this?

Error details>>

<iframe name="editorFrame" id="editorId" src="http://localhost:4000/iframe"></iframe>

// error was raised on
let iframeHead = iframeWin.document.querySelector("head") as HTMLElement;

// the server listens on localhost:4000
// the proxy code

app.use(path, function(req, res) {
      return proxy({
        target: "http://47.75.177.99",
        pathRewrite: {
          "^/iframe": "/",
        },
        onProxyReq(proxyReq, req, res) {
          proxyReq.setHeader(
            "Cookie",
            "tableau_locale=zh; workgroup_session_id=W2fKwkFeSzeC76d0hAAPmA|9vpJTvdHXW9VhaRrMLtwDzIazGYSgCIq; XSRF-TOKEN=NeRc6YfknJ5Vgj03xFwTQ30mftd1Jqlt"
          );
        },
        changeOrigin: true
      })(...arguments);
    });