I imagine a ring lock[1] alongside a traditional U-lock might be good as it would be quite difficult to remove without causing damage.
[1] https://www.axasecurity.com/bike-security/en-gb/products/Loc...
[1] https://www.axasecurity.com/bike-security/en-gb/products/Loc...
They look relatively easy to remove with a few tools in workshop. Perhaps you can simply lock pick them, with some practice.
But they do protect against casual thieves, the ones who just grab a bike for fun, or they are drunk and need to get home, or something like that.
I've had a ring lock on my bike for decades. But then the last couple of decades the bike hasn't been in a state where the pro thieves could get any money from it. :)
And actually, I think this is the best protection: Work on reducing production cost of bikes. Most of them seem ridiculously expensive for the material that goes into them.
The type of locks you'll typically find on bikes can be cut by $30 bolt cutters in about 5 seconds, which fit in a backpack, makes barely any sound, and you can bike away after. There's virtually no defence at night, police has to accidentally catch you in the act. If someone sees you, small chance they'll try to stop you over someone else's bike, especially at night when you're armed with a bolt cutter, and when they have no way to verify your story that 'it's my bike but I lost my keys'. If they call the cops they won't come, and if they do, will definitely not be in-time. And if they are, its a petty crime without real consequence, and the person will probably continue afterwards for quite some time.
Bike theft really is just quite shitty. The best you can do is make your bike less of an interesting target. There's typically lots of bikes around, if yours isn't the most expensive model and has two locks whereas other bikes have 1 lock, your bike is likely not going to be the one the thief picks.
And if you don't lock it to a fixed object, indeed it'll be susceptible to the bike gangs that just throw 20 locked bikes into a van at night and take the time unlocking them later.
Yes, some of my wilder fantasies include a large sprung spike in the downtube that could be released remotely when you discover the bike has been stolen.