I believe I've come up with a new build that works a bit better.
It might seem overly complicated at first as it's modifying four different stats but it fulfils the idea of widing the gap between non-crit damage and crit damage as well as keeping the boosted crit rate vs defense penalty.
1 Risk Stack increases Crit rate by 4% and Crit potency by 4%, decreases physical defence by 4%.
2 Risk Stacks increases Crit by 8%, Crit potency by 8% and decreases physical defence by 8%.
3 Risk Stacks increases Crit by 12% and Crit potency by 10%, decreases physical defence by 10% and the potency of non-critical hits by 2%.
4 Risk Stacks increases Crit by 16% and Crit potency by 12%, decreases physical defence by 12% and the potency of non-critical hits by 4%.
5 Risk Stacks increases Crit by 24% and Crit potency by 16%, decreases physical defence by 16% and the potency of non-critical hits by 8%.
So at 3 stacks, the Crit rate peels away from the defense penalty, but this is substituted by a decrease in non-crit damage.
This way the base potencies don't need to be low, as they're artificially lowered with more Risk stacks.
There's also more room there for melding Crit and Crit buffs.