A land value tax would help solve this unproductive utilization of land.
No, it wouldn’t—they’re leasing the land. (And the land is under an increasingly-useless mass of 20th-century office buildings.)
You won’t encourage landlords to not be landlords. You simply cut them out as middlemen.
Either landlords can make productive use of property within a proscribed set of regular or alternative uses beneficial to the public good regulated and adjudicated by the local jurisdiction, or they can choose to be penalized stiffly for idle inventory, absentee landlord, real-estate speculation that doesn't solve any local problems.
https://www.sftaxappeal.com/post/commercial-property-taxes-i...
The cost of these taxes is passed on in the rent.
The same goes for mortgages/rent etc. on residential real estate.
If you think raising these taxes might increase utilization/optimize revenues, great, but you’re proposing pulling an economic lever, not some genius new system.
The government has many economists and political scientists on payroll to determine optimal and politically acceptable rates, any changes would need much stronger and more detailed argument.