I did this ride just last week, had a great time. If you’re doing it southways, the bit between welshpool and builth wells is a long trek, I’d consider taking the major road for a few of those sections to gain some time. The cycle route always takes the minor roads or tracks but up down up down ALL day. The taff trail from brecon to cardiff is beautiful, reward day. After the LONG climb alongside the resovoir there are no more big hills and it’s pretty much tarmac off-road track all the way. The signage is mostly good but a bit hit n miss in places, and there’s a spot in a housing estate where there is road/track work going on and the workmen have just blocked the path without putting up diversion alt signs, very annoying when you’re trying to make a train in Cardiff. Take a map is what I’m saying and if you have a smartphone with google maps that would also be helpful. Also, if you short of time you can skip the last 6 miles and get a train to cardiff, costs just £3, route 8 passes right by Taffs Wells train station.
Have a good one.