Hello everyone.

I just bought a 2013 XT660X with 7500km on the clock.
Its completely stock, even has original tires on it with some pattern left.

The bike already had its 10.000 maintenance check according to the previous owner (and has receipts to back it up).

Is there anything else I should check on the bike before I ride it?

I heard the rubber cush drive gets worn out quickly, do you think I should be checking it already?

Any other advice for a new owner?

Hey mate, the Cush rubbers are easy to check. With the bike on its side stand, hold the top of the back wheel with your left hand then grab the sprocket with your right hand then give the sprocket a good wiggle to see if there is any play in it. If there is only a small amount of play then you could firm them up with Kev's Cush drive rubber mod using inner tubes. If there's a lot of play then it may be worth replacing them.
Other than that I don't know if Yamaha were as light handed with grease as they were on the Tenere's in the steering head bearings & swing arm.
Otherwise if it's only just been serviced & in good condition then just ride it & enjoy while you get a feel for it.