I have a similar setup in my front yard, no bushes, but I built a dry
stack rock retaining wall. I didn't worry about drainage because when
you dry stack stone there are gaps, plenty of places for water to
drain. I kept mine the same height all the way across. I didn't go to
tall though, I stacked 2-3 stones for a height of about 14-16 inches.
When you choose what kind of stone you are going to use keep in mind
that you are going to need a little extra stone to do the job. Don't
get just enough, because you need a selection to choose from. Laying
stone is a like putting together a big puzzle and without a selection
to choose from you will have a hard time finding the right pieces to
fit toghether nicely.What kind of stone are you planning on using? A
roundish fieldstone, or more square like Fondulac?