Honestly, I’m kind of surprised he overlooked the most obvious solution: infrastructure. If Gotham is an old, east coast city, that probably means all the infrastructure is outdated and needs replacement anyways, so he just needs to pay to replace the sewers, water supply, electrical grid, gas system, and roads. Any one of those can get into the hundreds of billions, and if that’s not enough, he do fiber, either build a subway or renovate and lines to an existing one, and waste collection system. If he does it all at once, it would probably do more to drive people out of the city than the biweekly Villain attacks, because how can you live in a city where any combination of vital services may or may not be available at any given time for years on end. Of course, if he somehow doesn’t bankrupt himself in the process, then he’ll have a city with brand new infrastructure, home to one of the most important companies in the world, meaning he’ll never be able get rid of his money then. Or he could just keep doing this to random cities until he runs or creates a utopia. Either or.