iPhone 6s iCloud Photos sync issue

I have a 64GB iPhone 6s. I decided to try to sync my photos library over iCloud. The full library is only on my MacBook and is about 80GB (35,000 pics). I have a 200 GB storage plan on iCloud.

Before I started the sync I had about 40GB of photos and video on my phone. I didn't think to delete these, as I thought (incorrectly) that syncing across iCloud would remove the duplicates already in the synced photos library. Now my phone fills up any available space trying to sync this large library, even though I have the optimization selected. I know about the need for 300MB of open space before the sync will start - the issue is that when I free up space to start syncing it is instantly filled. I can't seem to keep space open to allow for the sync to finish.

I think the solution is to delete photos from my phone to make space, but am not sure on how to proceed and don't want to permanently delete photos in my iCloud library. Because the library is partially synced (I turned off the sync option because it kept maxing out space on my phone), I can't tell which photos in Photos are stored on my phone vs. those in the iCloud library,and don't want to accidentally delete the iCloud photos. Questions to that end:

1. From Photos on the iPhone, how do you tell the source of a photo or video (whether it's iCloud or on the phone itself)?
2. If I delete a photo on my iPhone, and then set it to sync, will that photo be deleted from iCloud?

Any suggestions for resolving this would be much appreciated.

As an aside, I do believe that all of the photos on the phone are also backed up to Dropbox.
Thanks!

Sounds like you have quite a complicated scenario here.

On the assumption that all of your photographs are available either on your Mac and/or phone, I'd plug the phone in and let Photos do its thing with the option set to delete photos after transfer.

Then turn off iCloud photos on the phone and let the Mac upload photo library to iCloud. I've never done that part myself so if I appear to be sketchy on details, it's because I am.

Once your upload is complete, turn on PhotoStreM on the phone, but not iCloud library. This should make new photos available to all devices without filling up your phone.

Good luck. BTW, I like the idea of the DropBox backup. Seems sensible to have copies available on an entirely different system.