Hey Stathis - wow, you've been busy!
Cool creature you got there.
The modelling work looks pretty nice.
Couple of things. The muscle simulation is a good start but needs some finessing. Do you have your solver scale set correctly? It should be about half the height of your creature if your creature was the same size as a moose or something.
If it's set correctly, then the muscle materials might be too stiff. You shouldn't have to deviate from the defaults very much if your solver scale is set correctly.
What about muscle fibers, do you have them auto firing?
I've done a draw over of a couple of poses:
The brachiocephalicus looks like it needs to pull straighter when he (she?) reaches forward. This is causing a pinch which gets passed down to the skin.
The brachioradialis doesn't feel like it's contracting when the leg bends.
For some reason , generally, the muscles appear kind of stiff, or damped. It might be that they're over constrained. Are you using very stiff sliding attachments / fixed attachments between muscles? How many substeps are you running, and are you using any damping generally?
As for the fascia, there's definitely some cool sliding going on. You see it when you switch to the textured version. I'm wondering if you are using rest scale and pressure together? It seems like you're losing some of that nice detail that's sculpted into the fascia in your still image that you posted.
Without that detail, there won't be any details passed down to the fat / skin. There's probably an opportunity getting missed with getting some of that rippling muscle getting passed to the skin
While the wrinkling all over the skin looks interesting it might be better for focus the wrinkling that typically occur in areas that undergo a lot of compression like where the legs connect to the ribcage and the knees / elbows.
Thanks for posting your stuff! Hope some of this is useful