I'm going to assume you have a pretty good sized sewing room if it is over a two car garage, much larger than any room in your house. And you seem to be very happy with it. So I wouldn't trade a room in the house and send the guests to the garage (especially if there is no bathroom). I'm not convinced a shed, even custom built, would be cozy in winter with 4 walls exposed. Whether or not the cost would be recouped in sale depends on how long you plan to stay there - if you plan to be there forever, who cares? An addition on the house is OK, IF you want to be that accessible to the house (and occupants) when you are sewing. I am in the house and find I am easily and often interrupted.
I would be more interested in adapting the current space to your needs. A personal elevator may be in your budget, or a dumb waiter, or stair chair, or a set of stairs not so steep. And if you don't currently have a washroom, I would definitely see if it could be arranged! That way you never have to be in a hurry to get to the house! If you love the space and want to stay, there are lots of improvements you can do to make it even more perfect, and you don't have to pack and move or miss much sewing time!