Hate to dissapoint you. There are no best soil. What suits the tree best is depending on your care and the environment it is in. Many growers aiming for a soil that dries up in 24 hours a normal summer and adapt watering to conditions rest of the year.
Willows are very tollerant to wet root and could be planted in a more organic and water holding soil.
There are no ready to use bonsai mixes that kills trees. Just as long as you adapt care to the soil and tree they are all fine. I think its best to look into what is awailable locally. What leatherback uses in Germany is not awailable in Sweden where I am.
In Sweden the material is readily available as Leca or Leca kula. It seems that is the unbroken blahton in several gradings, including fine enough for bonsai. Probably if you know Swedish you can find it in a broken form, which I prefer to use.
No, I have searched and didnt found the broken form. I have broken and sifted some myself to try it for one tree. However too much work and too much waste. I also have one tree growing in mostly mini leca balls 2-6 mm. Still experimenting to find the best substrate for my conditions and care.