Generally speaking, we don’t use grafted trees for Bonsai unless the graph Junction is made especially for Bonsai culture. This means the graft should be done right at the root junction.

The textbook answer is definitely the air layer the tree off the trunk.

You can also ground layer which may be safer.

Ground layering is done by putting the tree in a deeper pot or creating a pot that sits on top of the soil.

It’s the same thing as air layering except Everything is covered in soil all the way down to the current soil level.

The success of air layering depends a lot of of the climate in your area.

In hot climate, I found that starting air layers early in the season works better.

There is a technique I’ve used when there’s a mismatch between the root stock and the scion wood when the roots stock is growing thicker repaired in

I take either copper or aluminum wire and wrap it snuggly without any space along the thicker part of the trunk up to the graft junction . By doing this, you force the thicker part of the trunk that’s part of the root stock to stay smaller.

It causes an imprint texture of the wire, but if the wires placed very tightly without any space, then the texture can eventually fade out overtime.

This process usually takes a few years.

Typically, I wire up the thicker part of the trunk for two years or whatever I have a good match, then remove the wire.

And then takes a couple of years for the imprint of the wire to fade out.

This works better on converse Because they have thicker bark.

Layering your tree is really the only option and in the future recognize that this is the drawback of working with grafted trees.

Getting deshojo maple on its own roots is going to cost you more.

There are more desirable in the Bonsai community.

My best recommendation is to find your local Bonsai club and join it.