My wet face is diluting my lather between passes!!

Initially I was mostly using the Badger and Blade guides for what soaps to use and as most of them had tallow I just went straight to the stuff I could find that didn't. But you're right of course, and I have some good, cheaper veg-based options to try from all of your suggestions now!
 
The key is to go at the soap with a fairly dry brush, and slowly add water once you have a paste forming on your brush. Then you work the lather until the bubbles are all but invisible to the naked eye, and the few visible are in little clumps of 4 or 5 bubbles. The lather will remain in a peak when you pull the brush out of the lather. This is your basic, good lather.

When the lather is at that point (and starting out, it's usually a minute of lathering more than you were expecting), you can leave it as is for a more cushiony lather, or add more water (just kiss the surface of the water with your brush) to thin it out and make it slicker. If you add more water before this point, you'll wind up with a runny mess, not a thinned out lather.

I've tried most of the various lathering methods put forward over the last 20 years, and they all pretty much boil down to the above, mostly with some extra steps, and without good visual clues to what point you are at in the lather making process.

For what it's worth, I face lather 95% of the time, and go to my face to work the paste into lather.
 
I've decided lathering is like golf. Unless you are a pro golfer, you nearly always tend to hit the ball short rather than long, by erring too much caution and forgetting it usually is as fine to be long as it is short (indeed designers know about this mental battle, so most danger is often actually short). Same with lathering - almost always err towards not quite enough water (and often not enough soap too). Taken me years to routinely use more soap, and even more water. Now to nail the golf....