Locks that work

That's what I thought but I even don't know which Kryptonite I should buy. I guess the Fahg will be to small for me even though it's more secure due to its size.

You mentioned an old (steel?) road frame. If you have such a bike, it should be no problem at all locking it with a Fagh.

The Fagh has the same shackle space as the Evo mini. I use an Evo Mini (living in a low crime area) for my beater. That bike has larger tubes and wider clearances than any road frame. If you fit the lock trough the seatstays, you can get away with a small shackle and still lock the frame + rear wheel to the bikestand.
Plus, the shackle will be completely filled, making it impossible to insert a lever or jack.
I can take a picture of the locking technique if you like.

Also:

I mean from the sounds of it, unless they really, desperately, must have your bike, put a fagh on it and they'll move on to an easier lock.

Completely agree with that. A combination of M18 and X54 is just overkill. If the thief is desperate, he will just cut the bikestand which is a lot easier then cutting two high-end locks.