I just want to let you know that while I can see you are concerned, I really don't think you need to be. If your plan is to wean slowly, you really DON'T have to worry about replacing feeds with Cow's milk OR your supply. I stayed home with my son until he was 14months old and we fed on demand that whole time . I could NOT pump to save my life after I stopped doing it when he was around nine months old. In fact, when he was 11months I was going to the spa for 8hours and I had to pump for TWO days to get 5oz.!! And then my cousin told me he wouldn't even take it! He drank water all day!
So fast forward 5months. I get a full time job and we begin trying to give my son cow's milk. He's simply not interested. I begin doing the research on cow's milk and despite what you hear from your pediatrician it's really NOT all that healthy for them. In fact while there is a ton of calcium in it, most of it isn't easily absorbed by humans. And he doesn't seem to like it so I don't push it. So I went from feeding him demand to being gone 8-10hrs a day 5days a week and while away from me all he drank was water. Ever. We would nurse when we together and on demand while together. I NEVER didn't have milk to feed him when I saw him. And you probably don't have a real supply issue either. The pump is simply no indicator of how much milk you are making especially when you have been exclusively feeding for a year. You ARE making milk and your body just doesn't want the pump to have it!
So it's very possible to start the weaning process and not replace feedings with milk and NOT worry. If you are still planning on nursing your child 3-5 a day at this point, that is enough breast milk not to worry. If you are still worried about calcium, again, the calcium from your breastmilk is much better suited and more easily absorbed than cow's milk. And other ways to get calcium are to eat what cow's do! So eat lot's of leafy greens or feed them to your LO!
And as far as fat, well the eating of cheese, yogurt as well has using healthy oils to cook in like Olive oil and feeding things like avacado should alleviate those worries. So you can begin the weaning process without worrying about your supply or your child's nutrition. My son didn't show any interest in cow's milk until he was two. And I have indulged it. He asks for "MOO" as he calls about once a week or every other. My son is thriving. He has grown almost a foot this year! He's 35.5 inches tall and a lean 28lbs. They really don't need cow's milk if you are feeding them a well balanced diet and are planning on weaning slowly! HTH!
ETA: If my son doesn't like something, I don't push it or try to "trick him". I feel like putting chocolate in milk is akin to putting sugar on veggies to get kids to eat them. I want my kid to eat things they way they are.