In that set-up, eventually that soil will scatter, and you will expose the roots, even in ordinary container repot, we keep the roots below soil level, not above soil level.
I would also trim the branches, so it does not go too top heavy. So the plant will redirect some energy to root production. Crassulas grow very slow. If you trim the branches you can easily make 4 or more bonsai set-ups.
The roots of succulents are softer and grows very shallow compared to regular trees that go into bonsai. With the trees, those roots eventually harden up later, so they get nice bonsai structure. But succulents are densely soft.
This is my little jade leaf I started 2 years ago: planted in a small bonsai container;
Jan 2013 - I started indoors, then moved it outdoors when temps are a bit better.
April 2013 -staying outdoors, so I keep the media very gritty and rocky, so if it rains it will drain well.
Jan 2015
Your plant is bigger, so I would put big rocks around it to help support it better. You may want to stake it for awhile to help it stand till the roots begin to anchor well. This way the soil is not too over the top.
Others may have suggestions, but that is how I would do it, and how I did my little Jade bonsai for now.
