I wonder if a vacant parcel tax would help here. Vancouver did this with varying success[1], and Oakland has this on the November ballot[2] If real-estate owners want to "wait" for a better deal, they should pay for the resulting blight (and speculation)

[1]: https://vancouver.ca/home-property-development/why-an-empty-...

[2]: https://www.kqed.org/news/11689739/oakland-wants-to-tax-vaca...