Waiting for a plot to be sold may take too long. How about an auction based pricing model?

Property owner declares land value and pays tax based on that value. Taxman has checks and balances to make sure that the declared land value isn't egregiously low balled.

The declared land value is made public and anybody can make an offer to buy the land at that value plus a premium. If the offer is accepted, all is good and we have a new declared land value.

If the offer is declined, a new declared land value is set at the offer price. The owner pays an additional tax to reflect the new declared value and a penalty if it is found that the old declared land value was (significantly) below prevailing land values.

The trick is in setting the penalty in such a way that it becomes unappealing to declare less than fair market value. The risk is that declared market value is unduly inflated to protect against the penalty, but the property owners self interest in paying the least amount of taxes should protect against that. Also applying a (small) fudge factor to the acceptable fair market value when the penalty does not apply should help. Markets do move and there is no need to (harshly) penalize normal appreciation and market developments.

When a plot is sold two different times (without change to the state of the improvements -- which is actually impossible, because agree it's a relevant part of the state), the change is likely to reflect the change in supply/demand of equivalent improvements in the local market (i.e., houses of X square feet with Y bedrooms.)