As Basil says, it will be lumpier the closer you want to go to the sea. I don't know South Wales too well, but Aberaeron to Aberystwyth is moderately hilly, but not particularly brutal. The coast route is also the main road, but main roads in Wales aren't necessarily that busy. Aberystwyth to Machynllyth is rolling on the main road; the more coastal route involves turning left at the top of Aberystwyth and is hillier to start with, but rolling rather than hilly.
From Machynllyth, the main road is hillier than the coastal route. Mach to Aberdyfi is rolling along the estuary, but then flatter toward Barmouth, across the bridge to Barmouth and then easy rolling up to Harlech and Porthmadog. If you take the Llŷn peninsular, the southern section is quite flat, whilst the northern coast is repeated sharp up and downs. It's hard if you have a lot of miles in the legs, but otherwise very do-able. The alternative is the Lon Las Cymru to cut off the Llŷn. Caernarfon to Bangor is Lon Las Cymru and is pretty flat (except the short section to the bridge). Anglesey coast is rolling, nothing massively challenging up the western side to Holyhead.
