Lowered marginal tax rates. Raising the marginal rates on the lowest earners is raising the effective rates on the lower middle class. That they don't get anything is essentially the purpose of your proposal.
> and high growth because you don't need to 'soak' higher-earning folks, who only pay moderately progressive rates.
But did you actually have to do that? Having the lowest marginal rates be in the middle is pretty expensive because it's also lowering the effective rate on everyone above them, or at best is just balancing out having the highest rates at the bottom. It seems like you're trying to increase the amount of the UBI while making sure the extra money comes from the middle rather than the top. Having approximately the top half (50% of the population) pay so that the second quartile (25% of the population) can get ~half the UBI instead of none both doesn't seem like a bad thing and doesn't seem like it would cost them that much rate-wise because it's a 2:1 population ratio and they they have a higher per capita base to apply the rate to.
And having the highest rates at the bottom is pretty bad incentive-wise.
> or pushes the break-even point way too high
What's the problem with the break-even point being somewhere around the middle? The people only slightly below that aren't getting a large subsidy, they're just not getting literally zero.
Meanwhile the amount of "well I didn't make that much money because I had half of it paid to my kid" marginal rate arbitrage you're reintroducing is large.