The ultra rich don't pay the current nominal income tax rate, because their income isn’t of the kind subject to income taxes, much less the additional taxes levied on labor income. The simple solution to that is to wipe away the distinctions about source of income in taxation policy (for payroll taxes, that means also eliminating limitations in source of income that qualifies for benefits and the quantity of annual income taxed, but that doesn't prevent, e.g., additional “bend points” in benefit formulas to limit the benefit impact of larger incomes.)
> Until there's an aggressive international agreement on closing tax loopholes, raising the taxes only impacts the barely-rich (doctors, lawyers, etc).
That's only true if you raise taxes on the kind and amount of income that doctors and lawyers typically make. Equalizing taxes on capital and labor income by doing away with payroll tax limits and favorable treatment of long-term capital gains while dealing with non-repeatable income (like most long-term capital gains for any but the ultra-rich) by allowing both advance tax recognition of anticipated gains and deferment of windfall gains over a period of years would impact the haut bourgeoisie, but have only modest, and mixed, impacts on the barely rich.