I took an alternative and quieter route off the main road for 10km between Garth and Llantwrtyd Wells - worked well.
My clubmate/ride partner and I also took the quiet very pretty and slightly hilliier route from Builth, most of it on Cycle Route 43 via Llangammarch Wells, and tirabad. The lanes . . . emerged half way down the decent to Llandovery on the A483 at Cynghordy. They were stunning lanes, worth the few more meters of climbing.
I took the 10km Garth to Llanwrtyd road last year but the guy I was riding with this year was unwilling to risk leaving the main road. It's about a km shorter but a bit more climb, on minor roads.
I like the look of the road the other side of the Irfon, on through Tirabad and btw, coming off at Garth, that route is a mile shorter than plonking down the A road via Beulah, yet only 15m more climb. On my 2020 'to ride' list.
I thought my (shorter but bit more climb) variation north from Llandovery on the minor road through Siloh/Porthyrhyd was superior in riding enjoyment to the routesheet/gpx route rolling down the A40 (at lunchtime) and then grinding up the long (6km) A482 climb, being passed by countless cars round those corners.
