This is a landscape tree. I hope it was not sold to you as bonsai material. My advice is to plant this beautiful tree in your yard where it belongs. If it cannot endure your winter climate, keep it in a landscape pot so that you can move/protect it in winter.
You're lucky to live in the USA where there are many hard working pre-bonsai growers who are developing a wide range of maple material that is ideal for bonsai at all price ranges, and they could use your support!
The nebari of your landscape tree is going to be very problematic. 'Fixing it' will either be impossible, or will take more time than starting with much younger material. Starting with younger material (or any pre-bonsai material) has two other advantages: you will have full control of the trunk-line, and you won't have that any unsightly grafts to deal with. There are ways to overcome the troubles that come with using landscape material, but they are a misuse of anyone's time.
Yes, you might just want to practice and find satisfaction in playing around with a tree like this, but you might as well practice and find that same satisfaction with appropriate material that will be much more rewarding.