A reduction like that is not something to be taken lightly. Look carefully to make sure there's a suitable branch that can be used as replacement apex and the reduction is likely to produce a better tree than current. Often the reality is not clear in photos on a screen so you need to confirm it can be done with the tree as it is in 3D.
I'm not a fan of wasting time layering sub standard material that's easy to get.
Consider that layering will hold up work for up to a year. That's time you could be spending developing the new apex and healing the chop.
Consider how hard it is to find another Chinese elm similar in size to the layer you'll take and how much suck a tree would cost, given the trunk will be straight and have little of no taper.
Now decide if the extra tree you'll get is really worth the time and effort and delay in getting on with the real tree. Everyone will come up with a different answer to that question so feel free to layer or try to strike a cutting with the redundant bits if that's what seems worth while.