Have you consider some part of that, since you are not in flatlands, but have some elevations to work with, to put the barndominium on a slope and build it as some kind of part bank barn?
If you can make the upstairs on some side have their own entrance at ground level and the barn it’s own below, that would solve many of the access problems dwellings over a barn tend to have.
I have lived in all kinds of places, some apartments over a barn and it was ok, except the time I was injured and those stairs were like climbing the Himalayas, many times a day, for every little thing, ugh.
It is super handy to live right where you can just walk directly in the barn, don’t have to first brave the outdoors to get there.
Sounds like you will have fun just researching all about this.