I’m not sure re 529 Garage or Bike Registry, but I do know that BikeIndex is easy for bike shops to search—we got one stolen Xtracycle back to its owner using it when I was working at the New Wheel a few years back. (It was a weekend, which meant it was impossible to check w/ SF Safe to see if the bike was registered with them or reported stolen—that site has no searchable database accessible to the public). However, I’ve been told that the police do check SF Safe’s list if/when they find a trove of stolen bikes, so I do list mine there too.
When buying a used bike, I’ve checked Bike Index (and in the case of my road bike, with the manufacturer as well, as I knew they’d have records....they confirmed that the original buyer hadn’t reported it stolen, and added me as the new owner of record).