Stop DSLR photos syncing to iPhone via iCloud

On your Mac you could keep your DSLR photos in a separate Photos Library and not add them to the Photos Library that you are syncing with iCloud.

I am only keeping a selection of my best photos and videos in my iCloud Photos Library, the items I want to have on all my devices. All other photos and videos are archived on my Mac in separate libraries, where I can access them, if need be.

Only one Photos Library at a time can sync with iCloud Photos, the library you designate as your System Photo Library, all others are strictly local, see:

• Create additional photo libraries in Photos on Mac – Apple Support

Like léonie (and probably at her suggestion) I also have a Favorites Library that connects to iCloud, and I have a Nikon Library that holds all my cameras' pictures. The Nikon Library has gotten large enough that I'm thinking of dividing it into a Nikon Archive Library and a Nikon Recent Library.

I import my Nikon pictures to the Nikon Library, edit them, and choose my favorites. Then I copy them to the Favorites Library that is my System Library.

To handle multiple Libraries, nothing beats the trusted 3rd party app PowerPhotos ($40.) PowerPhotos makes it easy to move pictures, albums, and folders between Libraries while preserving the metadata and organization. It also has lots of other features, like advanced Duplicate searching, batch editing of photo metadata, list views of pictures in albums and in the Library, and searches through multiple Libraries. They offer a free trial version.

With a MacAir being designed to be dragged around with you, it is awkward to keep an external drive plugged in. And because plugging and unplugging a System Photos Library can cause iCloud Photos to go crazy, you need to keep your System Library on the internal drive. So it becomes even more important that you keep the System Library smaller than your entire photo collection. The Archive Libraries can be kept on external drives and plugged in when needed.

This might be useful:

Understanding iCloud Photos - Apple Community

Turn off iCloud Photos on the iPhone, or at bets turn on Optimize Storage so the iPhone keeps lower resolution versions of the large photos locally instead of the full file.

Settings ➜ Apps ➜ Photos ➜ iCloud Photos and turn it off.

There is no way to pick and choose which photos get synced though. It's either on or off.

Alternatively, you can turn off iCloud Photos on the Mac to prevent it from syncing Photos to iCloud to being with.