I only played a lock a bit towards the end of BC for fun and just enough to get Affliction Mage tower done in Legion.
I dont know if the shard usage you speak of works like in Legion, but that is not how shadow worked in MoP / WoD.
see link for some vids that go into detail how Orbs worked.
I would recommend Preach’s vid on how orbs interacted with this vid:
Shadow Priest Basic Guide WoD
And in practice in a raid setting here:
Shadow Priest Basic Gameplay Guide WoD
And in PvP here:
SHADOW PVP MOVIE - ANBONI 1 | WOD 6.2.3
Those are good examples on how it worked and the PvP one is entertaining imo.
As far as Max level talents, I put this together here on my thoughts on how to make the differing playstyles work. Though I have evolved my thinking a bit since I wrote this.