Holiday traffic can make the M4/A48/A40 corridor a real bore.
A much longer, but better driving route to Pembrokeshire means leaving the M4 at Junction 25 Newport onto the A489 and A40 to Abergavenny and Brecon. Recommend Hills Burgers LD3 9SW for lunch.
If you’ve got time to explore, there are a few great driving roads heading north and south through the Brecon Beacons, but east to west you’re stuck with the A40. (The loop from Sennybridge south to Ystradgynlais on the A4067 and then back north across the Black Mountain on the A4069 is ace).
I would come off the A40 at Llanwrda and take the A482 to Lampeter then the A475 to Newcastle Emlyn and A484 to Cardigan. From Cardigan the coast road A487 to Eglyswrw then left onto the B4329 to Haverfordwest then back onto the A487 to St David’s and then back in a loop along the north coast on the A487 to Cardigan.
The best route back to the M4 from Cardigan is A484 Newcastle Emlyn then “over the top” on the B4333 to Cynwyl Elfed and back onto A484 into Carmarthen. From there, it’s east on the dual carriage way and then motorway, but keeping an eye out for cameras.
Most of the roads I’ve suggested are two lane blacktop. There are a myriad of smaller roads but they may be ace in a Mono but perhaps a little too exciting at times in your chosen steeds.
If you do get to Cardigan, drop me a PM and pop in for a drink.