On the GoC website, you create an account and link the 529 you’re funding with that account. Then you redeem the GoC gift card on the website and the funds get sent to the 529 (this does take some time, about a week).
I agree with your math.
It is entirely a personal decision based on personal circumstances as to the financial worthiness of this approach. I can say that as Hyatt Globalist last year we were upgraded to an amazing suite at the Andaz Papagayo Costa Rica over Christmas that based on cash price alone would have gone for nearly $20K. No way would that have ever been in our budget but we got to live as if it were for almost a week. And I used Hyatt points to pay for the stay anyway so it was very little out of pocket, just food and various activities we did.