Track Simplenews newsletter open per user
As you mentioned, you need a unique values for GA tracking pixel. You need to utilize Simplenews's integration with the Token module so you can insert unique values into the URL for each newsletter copy that goes out. 

For example, the inserting the token [simplenews-subscriber:user:uid] will let embed the unique user id into each tracking pixel copy. 

The tokens will vary depending on the details you need to track, but Token integration should provide almost every detail you need.