im by no means a security expert but i’ll go out on a limb and say the following
a big **** off chain on a cycle isnt necessarily the best advice.
some of the 16-19mm ones dont fit through the spokes and frames for a start.
also it might be less hassle to chop through your frame, so make sure it goes through your wheel as well.
i would also say a decent dlock combined with a decent chain is the way to go.
check out the paramount, onguard and kryptonite locks, many others but make sure its 16mm+ unless your just buying one to secure the front wheel, which is also good idea.
torc anchors and almax a good place to start, not cheap though
so to sumarise
decent chain to a concrete anchor
a decent dlock to another bike or object would be good (obviously in different place to the chain)
a mini dlock around the front wheel and maybe the chain if it reaches
that should delay things, but you want to stop them getting in as even if the first attempt is failed they might be back.
layers of security is better than one