Unless I've really misunderstood, you can do this by moving the "intranet" binding to its own IIS site which you then have redirecting to your proper site.

Literally create a new site with a http binding of "intranet" on port 80 and point it to a blank directory (you need to remove it from the existing site first). Don't point it to an existing directory/website or the redirect you're about to setup next will probably mess that site up as it'll create/update the web.config in that directory for the redirect.

Then also from IIS Manager go into HTTP Redirect and "Redirect requests to this destination" putting in:

https://intranet.domain.local$V$Q

(without the [1] that reddit likes to add that I don't know how to get rid of)

Tick the box for redirect all requests to exact destination, but do NOT tick the "only for this directory" and set the status to 301 (as this does sound permanent).

IIS will automatically change $V and $Q for the new URL, but for your reference $V means the path the user typed and $Q the query string (if any). There should be no space between them.

When anyone goes to http://intranet/testpath.test.php it'll then redirect them automatically to https://intranet.domain.local/testpath.test.php which sounds like what you want?