Get the chain and padlock as high as possible when locking it up so they can’t use bolt croppers effectively.
Mostly a myth, this- if the chain comes within a couple of feet of the ground then it makes little difference, you only need to get the tip of the cropper handle on the ground, and a bit of angle- and croppers that’ll break a quality chain are big, you’re talking 3 foot handles or more. And it’s pretty simple for them to raise the “floor” with something solid too. It’s not pointless, but it’s not something you want to put any faith in.
Secure chains are massive, it’s just inherent. Unless things have come on a lot since i last paid attention, there’s no chain under 13mm that’ll stop a big boltcutter and that’s really what you’re shooting for, it’s the professional thief’s tool of choice. But these big chains are useless for anything else bar home security, they’re just too heavy.
Anything less is for stopping amateurs and kids, which is fine but at the end of the day you end up with a chain that a thief can remove with boltcutters faster than you can remove it with the key (and can be cut with boltcutters that cost less than the chain)
And remember, Sold Secure, Thatcham etc have only 2 purposes- 1, sell chains and 2, satisfy insurers. They’re completely meaningless in terms of real security.