I have a similar situation with rivets, and handle it this way:
A complete part number for a typical aircraft rivet might be MS20470AD4-5 - the dash number indicating the rivet length. But I don't like to dictate to the aircraft mechanic what length to use, I feel he is the best judge to determine that to get the desired driven head diameter and thickness. So, what I do is call out the basic rivet part number that specifies the diameter, in this case MS20470AD4, on the drawing and the BOM, and then in the drawing NOTES, I have a callout "Install rivets per NASM47196. Fastener length to be determined at installation."
Elegant? Not in my vocabulary. But ya do whatcha gotta do....
The purpose of shims is for adjustment. So it would be hard to predict as what thickness would be required to fit into the place. One way would be to use smaller thickness shim (all of them of same thickness) and put a note for the quantity; use as required OR something similar. But other than this I don't think there is any better way to handle them. So your method is OK too.
I admire your desire to improve your drawings, but I don't think there is a better way than what you're doing now.
Thank you Gentlemen!