whether to cut up the stump or not
The problem with not cutting it up is that most likely it will split and check as it continues to dry. If you can saw it in 1/2 down the center with a chain saw that would work, or try to sell it as soon as possible. Another crafts person that might want it is a wood turner, who will want it to be in one piece, still fairly green so they can cut it up with a chainsaw and turn it on the lathe right away. Walnut develops nice color if it sits awhile, so maybe this piece is just getting better as it ages.