In a theoretical world, I would make the tax progressively larger after "financial independence" has been reached - either via step functions (every 1x over financial independence, 50% of the remainder is taxed) or some kind of parabola.
This may be too hard to explain clearly (even progressive commission plans are hard to understand!) so perhaps I would just settle for a higher tax rate.
My thought is if the founders want to make multi-millionaires out of the later engineers, a 50% tax will only do this in the extreme (10+ billion exit) cases. And in those cases, the founders will be billionaires themselves.
I'm assuming that Financial Independence is a large number (somewhere between 10 and 50 million) and that afterwards, the founders are in it "to build something great" rather than to keep score. Otherwise, you wouldn't consider the plan.
This isn't meant to diminish Detour. It's kind of like making fun of the first company to give equity just because equity can go down in value - I'm just making suggestions on a groundbreaking idea.