saving LR mobile to Mac

Your post is a little confusing, as it's not clear which app you are running on your Mac. You mention Smart Collections, which is a feature only available in Lightroom Classic, but you also mention that deleting photos from the phone also deletes then from your desktop, and that would only happen if you are running the Lightroom app on your Mac, not Classic. So which is it that you are using?

For information, there's probably no need to delete your photos from the phone as you can simply manage the space a lot better. If the photos were taken using the phone's native camera, then there'll be duplication when you import them into LrMobile, so once they have been synced from LrMobile to the cloud you can delete them from the camera roll (I assume you're using an iPhone?), but do not delete them from the LrMobile app. You can also use the "clear cache" option in the LrMobile app to remove the stored originals, which will be replaced by much smaller preview files (the originals remain in the cloud and would be automatically downloaded when doing things that need them, such as editing and exporting).

Many Classic users will manage their phone space (and their cloud space) by removing the images from the cloud AFTER they have been downloaded into Classic, as the downloaded files are stored on their local hard drive where they can be backed up by their standard backup routine. Once removed from the cloud they can be resynced from Classic back to the cloud (to make them available again on your mobile devices), but Classic only uploads much smaller Smart Previews which take up far less space, and significantly whatever space they do take up in the cloud does not count against their cloud storage allowance.

So, just clarify what software you are using on your Mac and we can help you work out the best way to manage your phone photos.