Here is my mastic tree progression.
Yes, its thin and doesn't have a great nebari, but i like its gum odor after trimming or watering.
I will work with this one as a mame bonsai in the future, maybe.
Be warned when you do start to remove parts of this tree that they aren't called a gumwood for nothing. It doesn't take too long to get sticky fingers and if you are doing any finer work take a bit more time to make sure you aren't stuck to a young branch or something and maybe keep a bowl of water handy to keep the fingers clean.
I have a largish Mastic in my yard and still not convinced that I want to pot it up but they grow very well in a mediterranean climate and there are some pretty cool specimens as bonsai around.
Maybe you first need to heal the chop first (taper the tree), and re-grow the primary branches after that, I'm not sure if the transition will look good in the future, it's a very flat chop. Keep in mind I'm also still a beginning bonsaïst, I'm just giving feedback what I think, it might work also this way.