To actually legally impose anything based on total net worth, you need to actually audit net worth and get a real figure.

So, what's wrong with that? You have a wealth tax on all wealth over $100 million. If you have wealth anywhere over say, $50 million, you hire an accountant to assess your business's value. Everyone with that level of wealth already hires accountants. It's a trivial additional burden. If your wealth is no where near the tax threshold, you don't need to bother hiring an accountant to get a precise figure.

There is a reason that every time such a policy targeting only the wealthiest is put into place (it’s been tried numerous times over the years in a bunch of European countries)

I'm calling bullshit on this. There are all sorts of taxes that fall heavily or solely on the wealthy. The reason the wealthy don't all leave is that they don't actually want to live in places that have low taxes. You can get low taxes in a war-torn hellhole, but most don't actually want to live like that.