I would probably do something like that.

const pathArr = pathname.split('/')
let nonExistentFolders
for (let i = 0; i < pathArr.length; i++) {
  if (pathArr.match(/\d+/) && i+1 < pathArr.length) {
    const folderName = pathArr[i+1]
    // validate id and folder name
    continue
  }

  nonExistentFolders = pathArr.slice(i, -1)
  break
}

for (const folder of nonExistentFolders) {
  // generate id/folder name pairs
}
// push new route

Have you thought of using next.js' property params in the page's component? It could first of all simplify the pathname logic.