I think you need a new value each year, otherwise of course if the economy goes well people will buy your land off you cheaply.
So you get to decide your tax each year but if you save yourself too much someone else will grab the excess value.
This game is terrible because even yearly is too slow. The property market went up 30% in my city this past year. If I set a fair price at the beginning of the year I’m forced to sell my house at a loss. Only if guess that I should overpay by at least 30% at the beginning of the year does my house remain unsold.
For a realistic implementation, why not just use the fact that the landowner already decided what the value their land has when they purchased it and/or paid people to develop it. Why not just use those values?