For John's route, stretches can be shortened by bashing down A roads (some trunk), so unless made mandatory, the route would need a number of info controls.
To get down from Upton Magna down to Hay away from the A49 we'll be able to enjoy the 'undulating' road via Bishop's Castle, Knighton and Clun, and then go over Gospel Pass/Llanthony (rather than Golden Valley) towards Monmouth/A466/Tintern. We can stay off the A40 too (at that time of day) from Abergavenny to Brecon, by taking the Govillon, Llangynidr/Tal-y-bont road on the south side. The Pembrokeshire coast road south of Newgale is not to be missed - but it'd be faster, easier and more boring through Haverfordwest to the bridge over the Cleddau. Carmarthen to Llandovery will be much nicer on the A4069/Bethlehem Road south of and parallel to the less attractive A40. And we can enjoy the Elan reservoirs and excursions into the Hafren forest (to Staylittle).