How to delete high-resolution images in the mac while keeping the iCloud version

Did you invest much work into naming faces and creating projects?

if not, you could simply copy your Photos Library to an external drive to have a local backup copy, then create a new, empty library on your system drive. Enable this empty library as your System Photo Library and enable it as your new iCloud Photo Library. Enable "Optimize Mac Storage" for iCloud Photo Library. iCloud will only download the optimized versions and keep the originals in iCloud. The only problem are the named faces and the projects, that will not download from iCloud. That is why you should keep the current version of your library as a backup.

See this page on how to create a new library and to switch between libraries: https://help.apple.com/photos/mac/1.2/?lang=en#/pht6d60b524

If you have a copy outside of pHotos you can delete it - the copy inside the Photos library will be optimized dynamically as needed if you have optimized photos set on your Mac - this process in not instantaneous but takes time

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this process in not instantaneous but takes time

And that is why downloading to an empty library is faster to get rid of the originals. Optimize will only remove originals if storage is needed and never free as much as possible. Downloading to an empty library will make the library as small as possible at once.