Deleting Photos for space

You could get a small, external SSD to move your Photos library (and any seldom used files) to and plug it in when you needed to open the library or access the other files. An excellent choice is this extremely, portable SSD - OWC 250GB Envoy Pro mini:

This way you can keep all of your photos.

Have you clicked Photos>Preferences>iCloud "Optimize Mac Storage?" That will cause the pictures on your MacBook to be smaller files of lower resolution, and it should save lots of space. The full pictures are in iCloud and available when you need them (for printing, for instance.) But "sync" means if you delete it here, it get's deleted there. You can't have "sync" and also be able to delete in one place without it happening everywhere.

Of course, you can turn off iCloud on your Mac, giving up syncing. You could delete anything or everything from your Mac Photos, and the pictures will remain on the iCloud website, as long as you keep Photos on your phone connected to iCloud.

Another options is to get an external hard drive and store your photos library there, removing the files from the MacBook completely.