I loved this a lot. I think you did a really great job implementing the risk. It is not optional and up to the player, but increasing the risk continously is the core of the gameplay loop. And my decisions were not so much about "do I take the risk", but rather: "what is the better risky play"? Risking to get a skull on the dice roll and lose, but preventing damage and therefore, being able to survive another turn if I do get unlucky with shields? Do I risk to take another extra skull as a reward so I can risk more, or do I play it safe and heal up? But, is this really the safe play or is it risk?
I think this is really well thought through and a great approach in terms of game design. You kept the visuals on a minimum and it totally helped to focus on the core mechanic.
Great work!