Importing my pics to make room on my iPhone.

This is what I would do:

1. Check to see if photos are optimized for storage on your phone (Settings > Photos > Optimize iPhone storage) If storage wasn't optimized, that will free up space
2. Ensure that your Mac is downloading originals (Open Photos and go to Photos > Preferences > iCloud and make sure download originals is selected) this ensure that you have your high resolution photos on your Mac.
3. Back up your Mac. If you hadn't been downloading originals you have a bunch of new data to back up
4. Toggle off iCloud photos on your iPhone. This will "break" the connection.
5. You now know that your originals are on your Mac, and that anything you do to your phone will not affect your iCloud photos connection.
6. Delete one picture (if you need to verify that your Mac doesn't update) and then delete what you no longer wish to have on your phone.

From here you have options. Maybe you still want to enable iCloud photos but with a smaller Library. Either create a new library in photos or delete images from your library. Otherwise you'll eventually have the space issue again

Maybe you just want to sync when you connect to your Mac. That's fine. you just lose the "immediacy" of having photos upload to iCloud. I think Camera Roll is still active so your most recent images may upload anyway (Haven't used it myself in a long time but someone else may be able to chime in)

But, the steps above walk you through your immediate storage issue.

Yes. If you're using iCloud photos and have your photos app connected to iCloud, then you'll need to be careful. Deleting a photo on an iCloud photos enabled phone will delete it everywhere. If you do not use iCloud photos to sync all of your photos, you can simply connect your phone, open the photos app, sync photos, and then delete them from your phone.