Looking for a bombproof chain lock rather than a D/shackle lock.
Ideally with a chain over 1m long. Will be attached to a bombproof ground anchor.
Bicycle-specific locks tend to be relatively pricey…better going down the ‘security’ or motorbike route? What do you consider ‘too much’ for said lock, given that it keeps your very expensive bike safe?

links need to be at least 16mm thick to stop boltcroppers as does the padlock shackle. Case hardened makes it hard to cut with power saws or to break by making it brittle by freezing. Lock needs to be double locking ie each side of the shackle

I would expect a couple of hundred quid to get something strong enough

https://securityforbikes.com/proddetail.php?prod=P19-x.x-SS65CS AT LEAST

What do you consider ‘too much’ for said lock, given that it keeps your very expensive bike safe?

Anything over whatever the insurance criteria are for payout. If they’re going to nick it, they’re going to nick it, or strip off whatever parts they can get at (I speak from the experience of someone off here who had a section of his garage roof removed so they could get inside, his bikes stripped for parts, taking everything other than the frames, which were secured with expensive locks, chains and anchors).

there are different grades of thieves though. Ground anchors and chains will stop a large proprotion of them.

Personally I would go for a chain with a big enough loop at the end for a really good d lock – belt and braces