There is nothing ''wrong'' with an S shaped trunk. Because mass produced bonsai are often trained this way, it is given the ''big yawn'' by seasoned bonsai hobbyists, but really there is nothing wrong with it. It is not a common ''natural'' form for an elm, but if you were to look at enough trees out in nature, eventually you would find an 'S' shaped tree. So as Judy B said, this tree can be made to look better, you can air layer off the top, or you can just work with the tree you have.
First, I'll offer a couple ''rules'' that are not rules, merely guides for when you don't know what to do. If artistically, something looks good that does not follow the rules, it is just fine. But if you are not sure what to do, the ''rules'' will help.
Branch order - this is not absolute - don't leave big gaps to meet this rule, but in general: left, back, right, or left, right, back, the pattern of branches should spiral up the tree. 2 right branches, or 2 left branches in a row is often distracting to the eye. Higher in the tree the occasion front branch is also needed, so left, back, front, right. You want to see the trunk at least half way up the trunk, and from that point up it is okay to hide portions of the trunk.
Outside of Curves rule - More important than branch order - branches should originate on the outsides of curves. Branches that come out from the inside of a curve almost always looks congested, out of place and awkward.
No more than 2 or 3 branches at any given point. On your elm you have a number of points where you have a cluster of branches coming off your trunk. In general, these should all be pruned to 2, the trunk and one branch. Or out on a branch, the branch should branch into only 2 sub branches. When arranging branches on the trunk, you need branches in all directions, initially left, right, back, then higher in the tree, left, right, front and back. If you don't have a nice sequence, of branches, if you have 2 coming off the trunk at the same point, one can be your left, and the other be your back branch, or right branch and back branch.
If you go through and prune off extra branches as I suggested, you can greatly improve the ''tree-ness'' of this bonsai without having to start an air layer.
An air layer will have to be in place about 3 months in order to root, during which time you should not do any pruning or repotting, just sun, water & fertilizer until the air layer is removed. An inconvenience if you are itching to do some pruning.