How about reduced stamina cost per roll as a capstone maybe -20%. (with dodge skill lvl 100 it would be -50% and with this rist it could go up tp -70%)

What if the reduced stamina cost was tied to executing a perfect roll, making the buff more skill-based? A perfect roll would reduce the stamina cost of the next roll by 50%.

"Flow-state" could be a name for the capstone

Both of these are about the same cost, so here is what the game already does with it: the Dodge skill halves the roll cost by itself at skill 100, and Tireless discounts rolling as part of move stamina. A third flat discount would be stacking on two. They also multiply rather than add, so a -20% card at Dodge 100 is -60% and not -70%.

So I am taking CRVD's version, and making it free rather than half: a perfect dodge makes your next roll cost nothing. Rist already knows when a dodge was perfect, because Answering blow hooks the same moment. Flow-state is a good name for it.

I not exactly sure how you intend to increase the invulnerable time during dodging, will the timer start earlier, or end later, or both, or will the animation be drawn out.