Yes, awesome yard you have!
Japanese Maples don't seem to love where I live too much. I'm think it's because we don't really get enough Chill Hours during the dormant season, might because it's also very dry where I live. I'm experimenting with humidity trays this year, but don't know how well they will do.
I believe the graft line is where the red line is. It's not like this is really bad or anything, maybe just something to take note of.
You absolutely could make Bonsai with the base of this tree, however my preference would just be to air layer a nice section of this and start over. I think in the long run you will have a better final product with the air layer. In the past I tried purchasing a couple trees somewhat similar to this one. I tried a low trunk chop with one and thread grafted the other. I did not really know what I was doing at the results were poor both times. However, even if you were to pull off a big trunk chop or thread grafts, there's not much taper in the bottom section of the trunk, not the best movement and or surface roots, plus you are dealing with a graft. For me, trying to make Bonsai with a tree like this was a good learning experience and something that I would not discourage you from doing. It's good to practice grafting, cutting, etc. and see the results are down the road.
I might suggest letting it recover, taking off some air layers, then trying to make Bonsai with the base section in that order. Or you could just do the air layers, then use the tree as a nice garden tree. If Shi Shi turns out to not grow too well where you live, don't get discouraged. There is a thing called Zone Envy, which is basically where you wish you could grow things that don't work well where you live. I try to stick with trees that do really well where I live, but I definitely have Zone Envy from time to time. Best of luck!