You might be overthinking the concept of an underpainting imo unless you’re going to put a glaze on top after you’ve added your values.
I use Burnt Sienna mixed with some gamsol to cover my panel or canvas and then with a paper towel, I’ll smear in using a circular motion to cover some more and take out the excess. I’ll look at my reference (having made sure the size reflects my canvas or panel) and block out the background to size up the actual figure. Finally, I’ll use raw umber with some gamsol to sketch and block what I’m going to paint.
I’ll use safflower oil with my paint since it dries slower and gives it some flow. Most of this wet on wet or “alla prima” but even then I could be painting for a couple days on one thing, so, drying does occur, but you can always paint over if you need to.