First, Shadow Sneak Attack doesn’t progress. It’s 1d6 forever.
You gain the sneak attack class feature, except you deal 1d6 precision
damage regardless of your level. Sneak attack from multiple sources
isn’t cumulative. If you have sneak attack from more than one source,
use only the highest number of dice when you sneak attack rather than
adding the amounts together.
However, fortuitously for you, it does stack with your Strategic Strike. Precision damage stacks with itself by default, and where there are exceptions, they’re called out specifically (as above).