Exciting to see the project coming to life.
The rib cage looks like it's twisting with each step, this shouldn't happen. Ribs should only be bound/parented to spine joints. If that's the case, then there's something strange going on with the rotation of the spine joints.
There are quite a few muscles that are going straight through the rib cage: The muscle that runs from the sternum to the radialis (pectoralis?), the lats, and the obliques.
I would encourage you to make a volume that represents the shape of the ribcage. Convert it to a tissue, and turn collisions ON for it. You can attach it to the rib bones, and turn collisions OFF on the ribs. This way, it will be a good representation of the thoracic motion, and it should give you a nice large volume to collide with. (Ziva is better at colliding with large volumes, as opposed to long thin ones, like ribs for example )
I think the animation could use another pass. Ziva simulations really sing when they're being driven by nice bone animation. I completely understand just wanting to get something moving though!
It would be good to pair up riggers with animators who want to do more creature-y stuff but aren't so focused on rigging
Looking forward to seeing the project develop!