I've played a bit with small thuja plicata that have shown up in my landscape. I've dug up these volunteers, potted them and played with making bonsai ranging from a few inches tall up to almost 24. At 24 inches tall and roughly an inch in diameter, the trunks are still pretty flexible, but only in the sense of 'with a length of rebar' - I bent the trunk 'around' a block of wood held against the trunk by a length of rebar, the ends of which were secured with padded wire to the base and apex of the tree. You may be able to do likewise, but be forewarned that it will need to remain in place for quite some time. Basically, the trunk will need to double in diameter to have enough wood to sustain the bend before the rebar-rig is removed.
I find that wound at the base of your tree to be most interesting. It could be THE bonsai feature of this tree, but to be that, I think the foliage would need to be much lower, so that it could frame it or draw one's eye to it. You do have some low branches that would make this possible - in other words, you could chop it down and go for a smaller/squattier image. But, my experience with small/short thuja is that they start looking like a palm tree = a stem with a puff of green stuff on the end that isn't the least bit appealing to me.
So, I'm raining on your parade
@Mara of dragon clan. Thuja can be extraordinary bonsai, but generally they have massive curving trunks that are more on scale with the foliage and its natural tendecy to hang in graceful sweeping curves. Nevertheless, I continue playing with my little stuff and admiring the real western red cedars (as they are known) in the local forests.
I think you should continue with yours and not give up. My message is only that it is challenging material for bonsai, so don't get disheartened. You can wire some stems and bend them into interesting lines. There is a trick of wiring it back and forth in the plane of sight so that the branch appears to be much shorter and the foliage closer to the trunk. One can also use something like #22 copper wire to actually wire the green foliage threads instead of just letting the foliage hang how ever it happens. It is one of the few species one would do something like this with, but doing it means one can make it hang in those delicious curves seen on the big ladies of the forest. I've yet to be satisfied with what I can achieve, but mastering this would enable successfully making a short (shohin) thuja bonsai. That and foliage trimming.
enjoy