Doesn’t the AirTag start making beeping sounds to alert someone they’re being tracked because Apple doesn’t want it to be used by stalkers? Even if a third-party makes a tag which is silent, would the thief’s iPhone notify them there’s an unknown tracker moving with them? It would defeat the whole point of using them on bikes, etc to catch thieves and recover stolen goods.

Android apps that detect AirTags exist too, e.g. https://github.com/seemoo-lab/AirGuard

They do, but people have of course realized that you can just destroy the speaker and then it won't.

But if you have a phone with U1 chip it will give you precision finding of unknown AirTags, making them a lot easier to locate than a bluetooth beacon.

See vanmoof.com Their bikes use the findmy network

definitely. I really only buy crappy bikes for that reason right now... a solid tracking option would do wonders