Don't bother with the train from Brizzol - I made that mistake a few years ago, when I lived there and couldn't get near a train from Temple Meads - and that was for Wales v Ireland!
If you're staying in Taunton, get the train from there - far easier and you may even get a seat.
If on the other hand, you're not all hitting the shant, then drive to Cardiff and park in the city centre - will cost you a fair packet, but easier to get in & out. Park & ride in Leckwith may be a good option for you. Not many people will be driving to Cardiff for the match on Saturday.