There is no potential for an eight foot tall formal upright bonsai there. You're talking about a garden tree, not bonsai.
Having worked with DAS before, I can tell you its frustrating and produces iffy results.
Digging this is a beginner's mistake. Size doesn't equal "nice material" Neither does a silhouette. Have you looked INSIDE that outline to interior branching. I bet its a mess of criss crossing branches that haven't been pruned in its entire life. "Trjimming" it won't get the job done. Major interior pruning with a had saw, or chain saw might get some of it accomplished...
This isn't to mention the root mass you will have to deal with once it's out of the ground, nor the soil you will have to come up with for its container. BTW, what are you going to put this in once it's out of the ground? If it survives and boils down to a bonsai, you gonna spring for the $1,000 + pot it will require?
Yes, I know it is a terrible idea. Risk/reward analysis is negative, it WILL be a lot of work (and some money) with negligible chance (approaching zero) of a decent tree.
And it still may not get done. Or I might get it home and decide that the best thing to do is to let it grow in the yard. Worst case I learn a little bit about collecting a large tree.
To answer some of your points:
The trunk is about 10" at the base, by classic Japanese proportions the goal should be a tree 5 feet tall. I think the top 2 feet will go away. 6 feet tall would look good.
Yes, the interior is a mess. It is a ton of long, skinny branches with a little foliage at the tips. The largest branches are about 3/4" and 30" long. I removed less than 1/4 of them to begin letting a little light and air into the interior. DAS do back bud a little bit, but not very likely. It also splits into 2 trunks about 2/3 up. One of those would have to become a jin. Eventually I would need to reduce the branches to maybe 12 or 20.
The root ball will be 5 foot or less. Currently is growing in a raised bed 6 feet wide from the foundation of the house. I am hoping to reduce the footprint to about 4 feet in diameter before we dig it up.
Initially I will build a wooden box for it. 48" x 48" x 24" Filling a container that large with something approaching bonsai soil will be my biggest expense, but doable. And even if the tree just dies I will use the mix for something else.
If it ever becomes something worth seeing, I would be delighted to spend a couple thousand dollars on a pot. Until then it can live in wooden boxes. Final pot would be about 24" in diameter.
I am not arguing or trying convince you, obviously you know way more than I ever will. And pretty much EVERYONE agrees with you, this is a terrible plan. I should quit thinking about it. Saw it up and make a violin. Go home and play with my horrible little trees and leave the poor thing to die in peace. Maybe someday I will have a couple of trees decent enough to distract me from stupid projects and making so many beginner's mistakes.