I'm having trouble thinking of a web stack where you can't set up a reverse proxy either via Nginx colocated with the frontend web server, through any of the usual suspects for termination (e.g. CDNs or load balancers), or worst case scenario, through the frontend web server itself.

Can you share any examples?

Note that if you send any non standard request headers, for the CORS response to be cached and reused, you need to make an initial request with exactly same set of headers, which means it is only possible via JS (preload request can't have extra headers). Hence inlined JS to send the "warmup" request. You can wrap it in a promise which resolves on response, and only then do the actual app request.

Just because they are on different tech stacks doesn't mean you can't do this.