True indeed, it's not possible at the moment to buy a game, that you have already been claimed. You would need indeed to cancel Stadia Pro, wait until it expired, then purchase the game. You can also subscribe again to Stadia Pro at any time.
I have read this in the past often as well, but I think best is if you let the Stadia developers know as well by submitting feedback that you would love to see that option (simply tap your profile picture in the top right corner on stadia.google.com or in the Stadia app and select "feedback").
