It is a horrible situation for you and I am very sorry for you and your family. <br /><br />That being said, I think *and this is 100% only my two cents*…
Now is the time to make other memories with your father. Go on trips together, watch games together, go out to eat together, play cards, take a cooking class, book a spa day for him- whatever! If you’re feeling an anxious “expiration date” there is 1,000,001 things you can do with your father that has nothing to do with your relationship to your SO.
Putting an ultimatum on your SO for this situation is unfair, in my opinion. He sounds like he’s always been very upfront in the fact he wants to wait- and until now you said you agreed. Everyone is allowed to change their mind- of course!- but not only changing your mind on something you both agreed on but then saying if your SO doesn’t also change his mind you’re going to leave is a bit silly and somewhat “cut off your nose to spite your face”.
Putting an ultimatum on him is not going to give you the magical wedding experience you desire- even if it does end up making your SO propose because the key word there is make. You’d never know if he did it because you threatened him or because he thought of it all on his own and felt ready. He may harbor resentment in you for not being fair and understanding his own reasons for wanting to wait- for saying his reasons aren’t as valid as yours.
Also the end of the year is in ONE MONTH. Not only do I think the ultimatum overall is pretty selfish, you’re only giving him one month to decide?
In the nicest way possible I think you need to seperate your fears from your relationship with your SO. It’s 100% valid to be afraid of losing your father. But there are things that you can do to somewhat allay the fear of not having these magical memories with him while he is still well- just go make them! Throw a wonderful Christmas/Hannuka/Holiday/New Year Eve party. Take him on the vacations you’ve always talked about but never have done. Or go back to places from your childhood that meant the most to you! There are things you can do with your father that wont force something inorganic and potentially toxic from your SO and your relationship.
Also assuming your SO caves and proposes… weddings are super stressful to plan (AND EXPENSIVE!). You may not get the ring you want in only one month with no saving or planning from your SO. Or if you would like a large proposal one month is not enough time to do that so something else would not come up to snuff.
You may not have the emotional and logistical planning help you’d like from your family because they may be too busy doing things with and for your father. There may not be any money left over if you’re planning on financial assistance from your mother and father. This could breed a lot of hurt if you start to feel like no one is taking your big day seriously- even if they have valid reasons for being aloof or apart from the planning.
Also you may regret spending this time that you have planning and being stressed and being too busy to ultimately have ONE memory moment with your father where, instead, you could be saving the time and money for your future with your SO and to spend it on making MANY memories with your father now.