I've just done Reading to Llanidloes "on road but." over four days.
The "but" being the route minimised, as far as is possible, any time/distance on A- and B- roads, in the style of Lost Lanes. This made for some really challenging climbs, some interesting surfaces, and very little interaction with motor traffic. In other words a delight.
Am now planning a similar "Lost Lanes" style tour up the Wye Valley from Chepstow to Llangurig and then down the Hafren/Severn from Llanidloes to Brizzle.
Anyway you wanted a concrete idea.... The Pennine Cycleway and Lon Las Cymru would be my concrete recommendations.