Hi Stuart
I may be a bit late commenting but all I can say is that the direct route the M4 is anything but scenic.
We are staying 3 nights near Pontypridd next week which is not far from the Brecon Beacons which has some great driving roads and possibly half way to Pembrokeshire if you can go that way.
We stayed in this Hotel last year and its great value on booking.com at the moment and very quiet location for leaving a Mog outside.
https://www.llechwen.co.uk/