I am the only one likely to say Yes, bonsai away. That is an Arborvitae and not a usual subject for bonsai or more realistically in this case, Niwaki or Cloud pruning. It won't fill back in after you bonsai it. That is the risk. Removal and replacement if you don't like the result isn't the end of the world.

And by "bonsai" I think you really mean topiary -- pruning and forming it into artful shapes ...? Whereas bonsai would be stunting its growth in a container and shaping and pruning it into artfulness.

yes

I think you want to trim to expose the bark. Go ahead. I'd like to see the After photo.