To anyone else who had their payment stuck on pending, I got a reply from Steam support:
See you all in 2022We apologize for any inconvenience this has caused, and we appreciate the interest in our Steam Deck.
We can indeed see that you were able to make the payment successfully.
Our users' places in the queue is determined by when the payments were completed. We apologize for the possible delay these errors may have caused, though unfortunately, Steam Support can't make adjustments to reservation priority. However, rest assured you have a place in line for the Steam Deck.