Hey, okay thanks for the info.
My suggestion for you is to set all the materials back to default and scale the solver up a bit. I've generally found it helpful to assume that your characters are about the same height as a human, and then experiment with damping/density per muscle to give the effect that the creature is larger.
If you do those things, the muscles may still feel 'droopy' under gravity. To tighten them back up, create zFibers on all your tissues and then add a little bit of excitation that's on all the time, something like 0.2.
Don't forget to set your muscle fibers back to default values too (if you already have fibers in your scene)
You can still have your line-of-action still firing the muscles on top of the constant 0.2 excitation.
wow! 4000 pressure and still no valleys? I've never seen pressure that high, good to know it's stable at least. I have to say that's pretty surprising. If you turn rest scale back to 1, are you seeing any pressure at all? Maybe that second material on the fascia is taking precedence which doesn't have a high pressure value?
Substeps and collision point spacing sound like good choices