Yes, hairy, but (heading east) you can turn off after St Clears to go on the quiet road via Bancfelin and Sarnau, and after that there's a cycle path alongside the dual carriageway to Carmarthen - not particularly pleasant, but safe. There are cycle paths in Carmarthen to avoid the busiest of the roundabouts, and take you onto the B4300 which runs parallel to the A40 but on the other side of the Towy. It's much quieter. Then cut across via Dryslwyn. On the A40 from Llandeilo to Llandovery the traffic can be fast, but relatively sparse. I'll let others comment on the Fishguard to St Clears stretch because, if I've ridden it, I've forgotten it. But I think if you stick to the A40/A4300 it will be pretty flat. For Wales. Diverge, and it won't be.