There is no rub or complexity. They have only convinced you there is a rub and complexity because they have political and propaganda power, and they don't want to be taxed. Those without such power don't get to use the same excuses; we just get taxed anyway.
This pretending that we somehow can't know the wealth of people whose wealth is in assets, or that it "doesn't count" or "only theoretical", is the most basic nonsense. We routinely tax assets that have much less straightforward values in the form of property taxes, and we do it every year, regardless if those assets are bought or sold! Common sense valuation seems to work just fine if you are poor! But if you are rich, and the value of your assets is calculated by millions of people every millisecond on public exchanges, "we don't know how much it's worth, and the price might change before he sells it, so we can't tax it until he sells it". Right? So what if the price goes down in the future...what if the price of my house goes down in the future? Nobody cares; you get taxed on what it's worth NOW, just for owning it, because we need tax revenue. This practice is hundreds (thousands?) of years old. We just don't do it for stock, because rich people don't want it!
It is not a "daunting exercise" to figure out what assets are worth, but ESPECIALLY this is true for stock holdings. The number of shares they own is publicly recorded, assuming they claim ownership. And there is no price in the universe that is more accurate or widely-agreed-upon than the price of securities on major exchanges. There is practically nothing in the financial universe that is known better than the price of publicly-traded securities.
Of course stock prices fluctuate, but we already value stocks all the time for donation, estate, and tax purposes by using price averages, or using the "greater or lesser of the price at the start or end of the period" or any number of other methods.
I'm only pointing out the incredible double-standard that comes up any time somebody proposes a wealth tax. But as a policy, both "wealth taxes" on capital, as well as the way we usually calculate property taxes (usually based on actual improved market value), are not very "good" taxes because they are a tax on capital, which is a product of labor, which is economically inefficient and has the effect of suppressing labor and driving up rent. Instead, tax-the-rich schemes should be calculated based on imputed rental value of scarce assets or land held. Calculated that way, they would be a tax on Ricardian rent, which would be economically neutral or even beneficial compared to taxing capital. I think we should tax the rich, but we shouldn't do it by taxing their capital, we should do it by taxing their land and rent. But pretending that taxing capital is "hard" is BS. My local county assessor seems to be able to figure it out every year when he drives by, looks out the window at my house, and decides what I will be paying in taxes!
More on land / rent taxes as wealth taxes:
https://en.wikipedia.org/wiki/Land_value_tax
https://journals.sagepub.com/doi/abs/10.1177/048661349402600101
https://www.weforum.org/agenda/2022/03/land-value-tax-housing-crisis/
https://www.elibrary.imf.org/configurable/content/journals$002f001$002f2022$002f263$002farticle-A001-en.xml?t:ac=journals%24002f001%24002f2022%24002f263%24002farticle-A001-en.xml