I saw someone on YouTube using compressed nitrogen or whatever it is that freezes when it's sprayed like Co2 canisters.
Freeze it, then heat it immediately, then freeze again, then heat again, and it cracks right away.
This is on Nitro Cellulose paint.
Polyurethane though, I doubt it'll work.
As previously stated, a razor knife, but if you slip, or cut wrong what do you have left? One ugly mess that looks faked.
Plus it'd take a ton of time to really get it to look real.
I'd stick with methods that give a natural looking checking, rather than anything that requires artistic touch, & can be messed up with ease.
As far as yellowing, there's probably a TON if different ways to go about this.
Put it in a smoker type box, and burn off a ton of cigarettes. Clear it when you're done.
Give it to a smoker who lives in a small apartment.
Or maybe a yellow/amber colored wood stain?
You could try yellow/amber ink from an art store?
Food coloring would probably work on bare wood, then stain?
Idk, just giving some random ideas.