I'm so sorry, but I don't think it will work that far from now.
My mom is a nursing home director of nursing, and she specializes in dementia and memory care, and I've volunteered in her home for years. As much as it sounds like a wonderful gift, and it comes from a wonderful place, people who are being enveloped by dementia are desperate to cling to things they remember. If they are in unfamiliar surroundings, it can be extremely upsetting for them. They are trying to hold on to the most basic things, like who they are and who you are and where they are and why you are here. Putting them in a new situation with radically different things, exactly what you want out of a vacation, can make them depressed, anxious, upset and combative.
I know what you want to do. My grandma had severe dementia in her last year. I wanted to see her so badly, but she was regressing and losing us so quickly. When I talked to her, I couldn't mention my son because he is named after my father, who died when he was 22. Grandma didn't remember that, so when I talked about Joey, she thought I was talking about her son, and it confused her and then she would realize that there was something wrong with what she was thinking and would struggle to get it back, and I think she knew that whatever it was, it was painful, so it made her angry. For her sake, and admittedly for mine too, I didn't come back to see her before she was gone. I didn't want to be the one who made her remember that her little boy was dead. She deserved more peace than that.