"I have had great success over the last 4 years with my new soil mix. 1/3rd crushed leca balls and 2/3rd chopped coconut fibres."

How would you compare leca with akadama?

I use my standard bonsai soil for my two willow bonsais, but I let them grow without any continer, but sit the root pad in a tray of water. The trees consume all the water during the course of the day in summer.
Since you can practically root any branch, you can pick a segment of branch that will result in an interesting trunk.

I'll add a photo, if possible, in my next post.

You can see the root pad. There is no container, and these grow continuously, so I occasionally trim the perimeter of the pad as-needed.

Here is the willow's progress after 2-3 weeks:

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Do you I have enough roots for planting the big willow?
Do I need to anchor it given that it's quite large (70cm)?