Welcome to the Lightroom Community. I'd love to help you figure this out.
Lightroom does not have selective syncing at the moment. It stores everything on the cloud & relies on the cloud, which makes all your work accessible everywhere.
In an experimental workflow, you can sync images from Lightroom to Lightroom Classic on the MacBook.
Lightroom Classic downloads full-size originals to the desktop from the cloud. Once downloaded, as a test, you can try deleting an image from the Lightroom Cloud (https://lightroom.adobe.com), or Lightroom & Lightroom Classic will still retain the original full-size image that is kept only on the MacBook & not on the cloud.
This might help you to some extent,