Testors paint

Acrylic or enamel? With their acrylic line I often notice transluscent one coat hand brushing coverage, no matter how well I stir it.

It sounds like (1) you are not using the mixer correctly, or (2) the mixer doesn't do the job.  I have a Badger cordless paint mixer. I no longer use it because my Tamiya (manual) stirrer does a much better job.

Ditto on the Tamiya stir stick. I swear by mine for its dual usage as stirrer and measuring device. Which colors are you having the transluscent ceoverage with? All colors, or just some lighter colors? Yellows and whites are usually problematic from all brands.

As simple as a tool can be. The blade end for stirring, the spoon end for measuring drops of paint into your mixing jar for nice precise ratios.

Go to McDonald's and grab hand full of their free plastic coffee stirrers.  Same thing.

One nice thing about the Tamiya stirrer is it can be used to test if a paint is correctly thinned for spraying.

Dip it into paint. Take it out. If you cannot see the edges, the paint is too thick. If you can see the middle part, the paint is too thin.

I don't know,I just use the the other end of the paint brush to stir my paint in the jar

I have used all sorts of implements for stirrers- old paint brush handles, pieces of sprue, old ink pen barrels, etc. The advantage of these things over those is 1) rigidity the metal wont break, or bend on more difficult older stirring challenges 2) the blade end works good for breaking up difficult clumps of paint pigment, 3) these things are not expensive, 4) the little paint spoon/scooper is really handy and easy to use once you start using that method... 5) they last- I only had to buy a replacement set after my original set disappeared from my work bench while I was away from home on an 18 month  mobilization and deployment. I have had this set in the nearly 8 years since I have been back.

I was thinking about using a little screwdriver - I got a whole bunch of those - but that spoon end is money! I'm gonna have to pick those up

With the Testors enamels I have the opposite experience.  I have a standard thinning ratio for airbrushing that works with new paints, I can use new stuff out of the bottle without thinning for brushing (though some thinning makes it work even better).  However, I find the paint begins to cure once the bottle is open, and the more times I open the bottle, the more thinner I need to add, until finally it will cure completely in bottle and need to be discarded.  No amount of care in sealing seems to make them last forever once they have been opened a time or two.