Many of these things are already valued, including some by the government such as lot of real estate. We can work to use those valuations. We can make it so insurance companies need to report valuations to the government for example. A painting worth 7 figures is going to be listed in some insurance policy. Report that number to the government and there is no legal argument to be had. Either it is being overvalued for insurance fraud purposes or undervalued for tax fraud purposes. The conflicting incentives should yield a relatively accurate valuation.

We also don’t need to make perfect the enemy of the good. We don’t need to account for absolutely every piece of wealth if we cover the big ones. We can also continue to revise these tax policies as new loopholes are discovered and exploited.