When to tell current employer that I want to quit and switch to a different PhD
I wonder if the choice is as stark as you think. It ought to be possible to begin a search for a new opportunity without advising anyone of your search. You can do this informally, of course, with other professors at other institutions, but also formally. 

The time that it will be necessary to make your desires know can be fairly late, say when the new place requires letters of recommendation and letters from other sources won't do. 

In some labs it is required to release employees at the first hint they might want to leave. 

But if you can line something up that is pretty firm, subject to letters, you can finesse the decision even then. "I just got a fantastic opportunity and wish to follow it up", rather than "I've been looking for something else for a while now...". 

But even if you make a commitment to leave, don't neglect your current work. Good work will make those letters better and easier to obtain. 

But, you won't really know how employable you are until you start to look and see what feedback you get. 

It might be necessary for you, in the short term, to let prospective supervisors know that your search is still confidential. I think most people would respect that if you give them a reason.