The suggestion that I've heard to solve this problem is if the wealth tax applies to you, you have to offer to sell your property to the government (not all of it, but you could claim so much under a threshold that isn't being sold, this isn't the full idea).

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.