Thanks for asking. I did indeed make it to Cardiff for the match! It all turned out to be pretty easy thanks to a lot of the advice that I received here.
I downloaded the app for the rugby stadium and was able to purchase a ticket secondhand through their exchange, and I was able to buy a GWR train ticket on the GWR app as well for a fairly reasonable price.
I took a train from Paddington at 945am - I arrived at Paddington about 25 minutes early and the track for the train had not been announced yet
So when the posted the track, I was able
To beeline to my train and grab a seat for the 1h 51m journey. I was lucky because a number of people had to stand.
Got into Cardiff around 1140 and was pleasantly surprised at how compact the downtown was, and how close the castle was as well. I wandered up the main road and bought a ticket to explore the castle walls and keep. This turned out to be a good purchase because i returned 2 times later that afternoon to use the bathroom :)
Once I was done exploring and emerged back into Cardiff, the pubs and atmosphere was all in fulll rugby day swing. The pubs were jammed and I grabbed a few Guinness at a pop up Guinness bus/bar before heading to the match.
The atmosphere in the stadium was amazing. The anthems before the game were really special. And prince William and princess Kate were there - seated directly across the stadium from me.
I am very thankful for the advice on this forum regarding the crowds for the trains after the match. I left with about 8 minutes left in the match (England had the game firmly in hand) and got into the queue early enough to be able to be on the first train out of Cardiff - and luckily enough to secure a seat for the journey. As I was in line, I turned around about 5 minutes later and the queue for the train was so long that I could not see the end of it!!
The train departed before 7pm and I was back in Paddington by 9 and at my hotel before 930. It was a truly special day overall, and I recommend it to anyone.
I was sitting next to an older welsh gentleman at the match, we got to talking a little bit before and during the march. He’s had the same seat for 50 years throughout the various incarnations of
The stadium. I told him it was my first match and we got to talking about the US as well as Wales. At one point he said something to me in Welsh, and in English he told me “Welcome to Wales”. Really nice man who I will not soon forget. The entire stadium was full of energy, but everyone was very well behaved and respectful.
10/10 experience.