Closer to the date, when your significant other is sure they will relocate, absolutely sure, I would request a 1-1 chat, face to face, to explain the situation.
Keep the discussion open ended. Not phrased like "I'm sorry I'm leaving" but "This is the situation, I'd really appreciate your advice and feedback". By keeping the conversation open ended you can seek win-win options with your boss that are good for you both. Stay in that mindset. If they try to close it by saying something like "you should have told me X months earlier" deflect them "this was a potential problem, but neither of us knew for sure and I am interested in what I do here and now". Keep the positive flow and offer something back, from negotiating remote work to staying with the company in a different location.
Stay dedicated at what you do, and when you're absolutely, positively sure nothing can derail the potential move for your significant other, have the conversation, and make it a conversation, not a notice.