Agreed about the maps/own route, the sustran ones were got as a default, but I was planning to do the odd diversion (road) as necessary. Any tips, or places to avoid on the official route (for a road bike)? Was thinking:
Day 1: Holyhead - Dolgellau (brother's mate lives there)
Day 2: Dolgellau - Builth Wells
Day 3: Builth Wells - Cardiff (in laws live there)
When are you going?
My advice is still to get OS maps Landranger 1:50k. You should be able to plot a nice peaceful route avoiding pretty much all traffic and seeing some pretty stunning scenery. I bought a Sustrans map once for the C2C a few years back. I didn't find it particularly detailed or helpful, let alone anywhere near accurate enough. Plus it sent me down roads I felt were too busy/dangerous. I wouldn't buy another much less cycle on any of their routes unless it was along a separate cycle route/path Dutch style or it co-incided with a quiet lane/road I had found on my OS maps. I have toured quite a lot in Wales and now have quite a few OS Maps. I love them and wouldn't use anything else save for a Satmap or Garmin Montana which both have OS 1:50k mapping on. But paper OS maps I love.
For route there is supposed to be a good coastal cycle path from Porthmadog via Barmouth to Dolgellau. Not cycled it I am afraid although Julia B has walked some of it as I was watching her more than the scenery

. Not stalking her but watching one of her excellent walking progs. Then from Dolgellau head for Machynlelth and from there you must take the mountain road up to Llyn Clywedog to join the B4518 going to Llanidloes. From here to Rhyader and then sustains route to Builth which is along quiet lanes except when you approach Builth Wells, but nothing too troublesome. Then from Builth down to Brecon and through the Brecon Beacons again use an OS Map to find quieter routes. It might be an idea to take a train from Merthyr to Cardiff as traffic volumes start increasing markedly unless anyone knows of any dedicated cycle routes?
Alternatively you could ride south from Machynlleth towards Devil's Bridge, Cwmystwyth then cut through the pretty Elan Valley across to Rhayader this way. This would be a nice climb on a road bike, not as steep as the mountain road from Machynlleth to Llanidloes but a longer climb and a great long down hill into Rhayader. Both good routes tbh. But take a look at an OS map for yourself. HTH.