I like the possibility of using the intense pigments of the High Load Acrylics because I am after color, not texture. But the possibility of marring is not appealing. What if I used high loads for the painting and then glazed it with a medium.
The High Load Acrylics have a higher pigment load, and are meant to be thinner than the tube acrylics but thicker than Fluids. They are like colored gessos. Just like a Black Gesso will show marring, these have a similar solids level and would do the same thing. They are really dense and rich, and if you are going to seal off with an "isolation coat" and a final varnish, there is no concern of the marring nature of them.

I'm looking for an acrylic paint that will allow me to paint flat, opaque strokes. Regular tube acrylics do not thin well, and the few brands offering more fluid paints are not adequate. I am currently using Liquitex Medium Viscosity Acrylics, which are the closest I've found to what I want. Do you offer something similar?
High Load Acrylics are heavily pigmented and are probably most similar to Liquitex concentrated colors. They are worth trying. We also have Matte Fluids. These paints mimic gouache, as they have a self-leveling nature, a matte surface when dry and very good opacity. The colors chosen will also have an affect on opacity.