How long for an HDMI cable to break in.

OP, keep the more expensive cable in your system for a few weeks, with signal running the whole time (just turn vol on pre) and after a few weeks swap back.  If it's still subtle or you are liking the old one, immediately return the $600 cable.  I would think there would be a difference right off the rip if it was higher quality but the connected components/system as a whole could make a big difference as to audibility of these types of cable changes.  I never change an audio component unless the change is not subtle, and repeatable.  If you have a friend to blind test you, even better (not that blind ABX is without its flaws but you should be able to denote the better cable most of the time)

Cables sound doesn’t change with use it’s a dead material. Pure copper is all that is needed which is better than aluminum which they use for over hanging electrical wires. Direction doesn’t matter light can travel in either direction no difference for it. The less connections and breaks you have in a wire is what makes it better. Also wires don’t know what they are going to be used for.

Why do you need 8K.??????

Most important thing with a cable is the sheathing.  There are plenty of other opportunities to waste money on audio gear the HDMI cable is not one of them.

Landscape look very carefully as to the data flow requirements going across the cable and you'll see that for audio it's minimal and for video it's far far far away from 8K.

It's not rocket science

More "Snake Oil"

If you hit it with a hammer and threaten the cable with more punishment, it breaks in almost immediately.