Synchronize clips settings

As you experienced, you can transcode the original clips, and if you work in proxy the proxy versions of your clips will be used. A sync clip has no media, it refers to the clips in it, so there is no need to transcode.

I can’t say without testing if it will make a difference but I would make sure the sync clip has the same dimensions as the original video. By making it small you risk scaling down from 6K to 720 and then scaling up to match the project. Depending on how this is handled internally it might or might not cause a loss of quality. Either test of just be on the safe side.

You don't need to do anything with the synced clip.

If you create proxy media for the video clip, it will be used.

That is easy to confirm.

When you edit in proxy, any clip that has not been transcoded to proxy will show in red.

Your synced clip won't.

Bottom line: make the synced clip the same size as the video, and edit away. No worries.

Try syncing the 6K clips and making the proxies with that.