My thoughts are twofold here.
1) I agree with you, taxing asset holdings is strange logistically.
I think it be best to tax income. The only change I'd make is currently, income is taxed at a percentage based on your total income in the year. What could change is to tax income at a percentage that is a function of the current estimated value of your wealth instead. So if you cashed out 1 million and that's all you have, you'd pay less tax on it than if someone cashed out 1 million but still had another 10 million worth.
2) Maybe it's a bad idea to allow anyone to own too much of anything of great value to society.
In that regard, it could make sense to force wealthy people to sell some of it, to whatever treshold we believe is too much for one person to own.
That's where I think a wealth tax could come in as a vehicle to force people who own too much to sell some of it. So that we have a more evenly distributed wealth ownership accross the board.
The only thing here is I'm not sure if a wealth tax is the best scheme for this. I think the income tax that I described in #1 would be good when it comes to taxes (money that goes to the government). For wealth, I'd be more inclined with something like where people have to sell a percentage, but taxes don't necessarily need to be involved (beyond the income tax as described from the sell). The idea here is just that no one should own too much, so at some point, you need to sell so that ownership is better distributed. Not necessarily that this should go towards taxes.