And then don't asset owners end up paying a surtax on their emotional attachment to their property? i.e. the only way I can hold onto something I would never want to sell is by paying a "wealth tax" that is decoupled from its market value.
Also if it's too much of a burden to value all these assets, you can list them publicly and let people bid. If you expect the government to protect your ownership rights, I think it's not too much to ask that you list your major assets.
Such a scheme could also require you to list a value of "all unlisted assets". If you try to hide a gold bar by lumping it in with all your trash, then the government can buy out and auction "all of razorunreal's unlisted assets" as a job jot. Companies would pop up specialising in identifying people with hidden assets to profit from them.
You always have the option of letting the government buy it from you for your declared value, and then buying it back at auction for actual market value, potentially making a profit if the government were wrong.
If the government believes that it's worth more than that, they are allowed to purchase it for that price, and resell it at auction. If they think it is reasonably priced, or over priced, you have to pay the wealth tax on your proposed price.
This will drastically simplify the problem. There won't be a negotiation on the price, there would be two independent evaluations, rather than one complex one where there's a large amount of back and forth.
I don't know where I heard it, but it was mentioned offhandedly as a simple solution to valuations for a wealth tax.