Carrier bearing preload is tough thing to achieve for a home-gamer, but I think you're right where you need to be. At the factory they have a special tool that spreads the rear end housing apart a few thou (those big holes in the cover flange are where the spreader pins go in to pry it open a little).
If you can just squeak that last side shim in place with a few light hammer taps, you're good to go. What you don't want, obviously, is there to be ANY side-to-side movement of the carrier possible.
I've averaged about 1 rear end setup a year for the last 12-15 years just doing it on jackstands on my garage floor (a few more while still installed on the car) so I'm definitely a "home gamer". Once an "old timer" taught me the trick of these things (I was relying too much on measurement, not enough on feel and contact pattern) I haven't screwed together a bad one yet.