I think the drop rate is not mathin'
this post is only made so you could remember to check
Suggestion -
I do think shards should drop from 1-20 mobs if you Earn experience (prevents making low level farm @ 35 from happening) not just 20+ mobs. just Extremely rare. .5% or something. and then whatever your philosophy on level 20+ drop rates
planar stones form 20+ feels good. IMO.