how do i keep my photos on my Mac but delete from my Iphone to clear storage?

Consider to use two different Photos Library on your Mac: On a Mac you can have as many Photos Libraries as you want, and you can easily switch between them. Only one of them can be your System Photos Library and sync with iCloud Photos. Create additional photo libraries in Photos on Mac – Apple Support (AU)

When I started to use iCloud Photos I set up two Photos Libraries on my Mac:

• A library for syncing my photos between my devices to be used with iCloud Photos: This library contains only my favorite photos and videos that I want to see on all devices at all times and is small enough to fit on all devices and into iCloud. I am also keeping the most recently taken photos in the iCloud Photos Library so I can work on them on all devices. My iPhone with the least storage determines the size of the iCloud Photos Library. I am trying to keep the iCloud Photos Library below 250GB.
• An archive library with all my photos on an external drive on my Mac: All new photos are copied into the archive library, and this library will regularly be cloned onto another external drive to have a backup copy. Over the years I split the archive library into several libraries, one library for each year. The reason for this is, that Photos is needing a lot of background processing to analyze the photos, which will take several days or even weeks, and this has to be repeated after each major system upgrade. So smaller libraries are easier to use.

To set this up, do not disable iCloud Photos at all. You will want to keep your devices syncing to transfer new photos and the adjustments and metadata between your devices. You just want to weed out your iCloud Photos Library.

To get started with archiving your old photos on your Mac, make sure, that all original photos and videos have been downloaded to your Mac. Otherwise the library on your Mac will be incomplete.

• On your Mac:
  • In the Photos > Settings > iCloud: Set the checkmark "Download Originals", if it has not been enabled all along. Watch the status bar until all originals have been downloaded to the Photos Library.
  • Find a suitable external drive. It has to be prepared as described here: Move your Photos library to save space on your Mac - Apple Support Not all externals can be used for photos. The file system needs to be compatible and do not even think of using a NAS: The external needs a wired connection.
  • Quit Photos, if it is running. Then drag the Photos Library to your external drive. Double click the copy on the external drive to open it in Photos. If you are seeing a warning about "Incomplete items" that need removing, there are still missing originals and you should switch back to the library on your internal drive. work with the older photos to force a download, then copy the library again and test it. I would make the first backup copy of this library
  • Now switch back to the iCloud Photos Library on your internal drive. As you are having an archived copy on the external drive, you can now start to weed out your iCloud Photos Library and delete redundant photos and videos, that you do not need in iCloud and on all your devices. as the library is still syncing with iCloud Photos, any photo you delete on your Mac from your iCloud Photos Library will be deleted from iCloud and your iPhone as well. Keep only your favorite photos in this library and remove all duplicate shots or mediocre photos, that you would not show to anyone anyway.
  • Empty Recently Deleted regularly, as long as it is not so full, that you would not notice, if you accidentally trashed an important photo.
• On you iPhone and in iCloud, check if the deletion is syncing to the iPhone and you have freed storage.

An added bonus of keeping the iCloud Photos Library small and focussed on only our most important photos is, that it will be much easier to work with it. Photos will be faster after a system upgrade, when the library needs to be scanned again. The albums will be smaller and easier to navigate, it will be much faster to find the important photos we are looking for, particularly on a small display like the iPhone display. And when we want to show our photos to someone, there will only be masterpieces in the library. It is much more impressive to show just ten perfect photos than having to browse 200 photos to find them among all the redundant photos.

if you want the photos removed from iCloud, there is no need to remove them from the iPhone

you need to switch off sync to iCloud for photos on your iPhone AND on your Mac

When you switch it off, you are asked to keep them on the device or on iCloud, read it exactly and remove them from iCloud only

I turned this recommendation into a new user tip: How to keep the iCloud Photos library small and archive other photos locally on a Mac… - Apple Community

Now that we no longer have My Photo Stream, there is no other easy way to sync photos from our Mac to the mobile devices, and iCloud Photos is all that is left for a wireless mass transfer of the new photos from the Mac to the iPhone or iPad. Using the Finder for syncing will just transfer low resolution versions to the mobile devices, that cannot even be deleted directly from the device, and it is just one-way.