I have some experience with thuja, mainly old collected specimens. Those don't bud back reliably. Definitely not on the trunk, but maybe in the crotches of the branches if you are lucky. They do bud back reliably on the young growth (1-2 years old) with proper pruning techniques.
Yours seems like a younger specimen, but I still wouldn't expect any reliable back budding where you need it. This leaves you with a nice base, but several spindly branches with foliage at the tips. The main trunk that was chopped will certainly die back. It looks like the strong part of the tree was cut off, and you are left with the weaker bottom branches which will need some time to regain vigor.
Thuja are a wonderful species for bonsai - don't let anyone tell you otherwise. But my honest opinion is this is not the greatest starting material you have here. If you do get lucky with some back budding near the trunk, you may have some interesting material for carving.