Where I live there's a stretch of our old downtown that is a block of buildings owned by absentee landlords. The buildings are literally falling down because the owners are probably using them as a tax write off. There's no reason they should get to sit on that forever.
I think stricter government controls over not allowing buildings fall apart is a better solution than cranking up taxes, or trying to take away the properties from the owners.
I rent out a small apartment I used to live in. Rent is below market rates cuz the little old lady is the best tenant I could hope to find. Raise taxes on that place because you fantasize that everyone is a slumlord, then I have to raise rents. Apply rent controls in addition, and that lady gets the boot cuz I sell it.
I don’t think your solutions are actual real-world solutions. I think they’re solutions to a perceived problem (evil slumlords) rather than what is happening in the real world.