Thanks for the suggestions. I’m thinking of doing the following now with a tent to keep the cost down:
Train to Bristol Parkway, then follow the LLC all the way up to Caernarfon then turn round and come back down the LLC using the alternate routes where possible. So A4085 from Caernarfon, inland route to Dolgellau, coastal route from there to Machynlleth, the loop through the Elan valley then down the other leg to Cardiff just for good measure.