Just back from that neck of the woods (half hour from Llanelli) and the route I took was M74 - M6 - M5 - M50 - A40 - A449 - M4.
Took around 7 hours with 2 stops. If we didn't have the kids and only needed a couple of comfort breaks, instead of proper stops, it would have been closer to 6 hours.
There's roadworks on the M4 between Newport and Cardiff with a few speed cameras along the way but not average speed cameras.
I would recommend the services on the M5 just before the start of the M50 (Strensham, I think). Quite new and has a Costa for a caffeine hit for the last leg of the journey.