Using Express Router to proxy non-CORS content doubles request path - proxy

Question

There must be something obvious I am missing. I have an external record like https/udspace.udel.edu/rest/handle/19716/28599 that is not set up for CORS, but I would like to get access to it in my application. In the past, I have set up a server to simply GET the resource and redeliver it for my application. However, I read about Node http-proxy-middleware and thought I'd try out a more direct proxy.
First, I could not use the target in the createProxyMiddleware() constructor because I wanted to send in the hostname and path of the desired resource like, perhaps, http://example.com/https/udspace.udel.edu/rest/handle/19716/28599 to get the resource above.
Relevant Code (index.js)
const express = require('express')
const { createProxyMiddleware } = require('http-proxy-middleware')
const app = express()
app.get('/info', (req, res, next) => {
res.send('Proxy is up.');
})
// Proxy endpoint
app.use('/https', createProxyMiddleware({
target: "incoming",
router: req => {
const protocol = 'https'
const hostname = req.path.split("/https/")[1].split("/")[0]
const path = req.path.split(hostname)[1]
console.log(`returning: ${protocol}, ${hostname}, ${path}`)
return `${protocol}://${hostname}/${path}`
},
changeOrigin: true
}))
app.listen(PORT, HOST, () => {
console.log(`Starting Proxy at ${HOST}:${PORT}`);
})
I was getting a 404 from the DSpace server without other information, so I knew that the request was going through, but transformed incorrectly. I tried again with an item in an S3 bucket, since AWS gives better errors and saw that my path was being duplicated:
404 Not Found
Code: NoSuchKey
Message: The specified key does not exist.
Key: api/cookbook/recipe/0001-mvm-image/manifest.json/https/iiif.io/api/cookbook/recipe/0001-mvm-image/manifest.json
What dumb thing am I doing wrong? Is this not what this proxy is for and I need to do something else?