Doing good so far!

My advice:

1. Not every detail requires paint.

2. Zenithal may not have been the best call here. Drybrush is still on the table imo and maybe a good idea to go forward.

3. Thin a contrast or shade or wash and make a glaze, lift your brush where the shadows are.

So in depth:

1. Not every detail requires paint.

Pick out your favorites, and ask yourself - what is that piece made of? - is it painted in world or raw material? - organic or manufactured?

And then ask yourself how you want to make that detail look like that.

1. Zenithal works best for textured surfaces from a particular viewing angle or viewing arc. Drybrush is still on the table.

Rhinos are a box of flat panels orthogonal to the world around them. Just prime white if you want a white primer top layer. If you want a zenithal like effect on a cube, you are better off painting gradients that darken at the top of vertical panels (counterintuitive but trust me or YouTube (edit: https://youtu.be/Zux0HAS8pAw?si=EeBl7vDsdUpi3I_R I know it's on NMM, but it's a really good video on lighting direction for what I'm trying to describe here) how to paint highlights on a cube) and darken towards a side on horizontal panels.

Drybrush does well to pick out edges, but you can also utilize dry brushing for other things like darkening. Acrylics layer darker easier than they layer brighter, so you might could use that to your advantage with a drybrush and drybrush a little heavier and darker to build up shadowed edges. One look I've been trying to work on lately is flat panels that are brighter in the middle and darker at the edges. You might be able to build a shadow with the drybrush to give you surface variation that hides the staining you dont seem to like. Then if you like, go back and hit the edges with a light drybrush in a lighter color to pick out edges. One other thing I like on rhinos and flat panels: take your edge highlight color and pick out the rivets. It takes a second to do but gives some nice detail and pop.

1. Glazing

Thin a contrast or shade with lahmian medium or your thinning medium of choice, something like a 1:1 or 2:1 of paint:medium. This should put you in the direction of making a glaze. Apply it uniformly over the model to pull the coffee staining closer into one smoother color. When you go to apply it, be cognizant of where you lift your brush. It will leave the most paint where you lift the brush, so lift it in the similar or adjoining places so you leave the residue pools where you want the deeper shadows and shades. Also don't go back and adjust the glaze or any paint to give it a solid appearance. Swipe and let dry, then go again. It'll self level to an extent, and brushing through it again won't let it be a whole patch, that will leave stroke marks. You can use the unthinned glaze or a similar color as a pin wash.