How can I remove photos after sync with my mac?

Hi, and welcome to the Apple Community.

Please take care here:

If I understand it correctly: Seems if you delete the Photos on your iPhone

the synced Photos on your Mac would also vanish.

Photos that are in sync on both devices are not saved (do not reside) on both devices.

They are just kept up to date with each other.

Edits/ deletions appear on both devices whenever they are in sync.

(When you Sync photos they are not actually transferred.

When you turn sync on .. it can also be turned off.)

To keep your Photos safe:

You would need to actually import your iPhone Photos to your Mac, if you intend to delete the Photos on your iPhone.

(It would also be a good idea to back them up to an external drive / jump drive or similar)

Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support

All the best :-)