LP640 TT on ADV7.1

You're right!

I've heard of drivetrain problems with aftermarket wheels larger than 18's.

wheel diameter size is almost irrelevant (sometimes tyre pressure sensors don't like it), it's tyre size that matters.

extra wheel width can cause problems by overloading wheel bearings and uprights, and on the front end, changing scrub radius can adversly affect the steering behaviour. etc, but it's usually ok.