Bonsai has phases where techniques change. To learn the full art, you need trees in each phase. My first thoughts on this, others may break out different categories.
*Seedlings
*Rough stock - nursery stock - maturing seedlings - sticks in pots
*Newly collected yamadori
*Bonsai ready for first styling thru to ready for first exhibit.
*Bonsai in refinement, nearing maturity.
*Bonsai, mature and post mature.
It is good to learn how to prepare young stock to become prebonsai ready for first serious styling.
Next is learning to execute a style through a succession of styling sessions.
Finally to complete a bonsai education it is key to get several trees in the refinement phase. Here is the most dramatic change in techniques. Instead of continuing to buy more sticks in pots, save up and get yourself a tree ready for the refinement phase.
Just a thought, it has taken me some 25 years to get my first trees in the refinement phase. If I had been willing to slow my purchases of young stock, I could have gotten there much sooner.
So this thought is just to save you time in learning the whole of bonsai.