If you want the single report from newman, I can’t think of a way to accomplish what you are trying to solve. If you are fine with multiple reports, just executing newman for each collection would have been the route to go.
If you have postman pro or higher, you could write a script that uses the API to give you all the collections in a workspace. You could then iterate over each collection from the response and run newman for each one. That way your test runner would always be up to date when you add or remove collections.