Taxing assets is indeed incredibly hard.
A tax system based around the logistics function (so that the tax owed is the definite integral between the starting point and end point, with no tax credits for anything, and the function is asymptotic to acceptable extremes at both ends) would seem to be the best system in terms of a pay-as-you-earn type function, because you don't get anomalies on boundary conditions or weird pay schedules, and you should be able to eliminate most of the cost of the tax rebate system.
However, integrating that with assets is hard. Which assets would be included? If all of them, then that would have to include properties, and property values fluctuate massively. If you only include "squishy" assets like stocks (where you can sell them quickly), it would be easier but you'd have to know what happens if the upper end of the integral is below the lower end. Do you pay back? If so, what happens during a market crash? If neither side pays anything when the tax integral is negative, is that something that can be exploited to form a loophole in the system?
I honestly think one of the S-curves would be the logically correct tax system (so every cent is effectively in a new tax bracket), but deciding what goes in there is not easy.