My parents have a house that's broadly that size (I think) and put ductless mini split heat pumps in about 8 or 10 years ago. They kept the oil furnace both as a backup and also because it provided the house's hot water. Overall, they love it. The oil bill went way down and as a bonus they now have air conditioning in the summer. It probably helps that the downstairs is relatively open, so they get away with one heat pump downstairs, positioned near where they spend most of their time. The upstairs is much less open, but their bedroom is the only room that gets regular use upstairs (they're empty nesters), so they have a heat pump in their bedroom and just let the other rooms upstairs run a bit cool (probably like 55-60°F). If guests come over and actually use the other rooms, they just use the oil heat to make them comfortable for the brief period guests are there. It's probably also very helpful that their house is set up so that basically every room has its own zone for the oil heat, so it's very easy to turn the oil heat up or down in just one room if it's needed for whatever reason.