We did a DIY from Aber-Cardiff last year. The route went via Carmarthen and I can probably find the autoroute track for that bit of the ride if you're interested. I think we did it as one leg, taking the shortest route via Llanrhystud, Temple Bar, Llanybydder, and Alltwalis. We also did a 300 DIY from Aber that passed through Carmarthen. This route headed down the coast road to Cardigan, then over Mynydd Preseli to cHaverfordwest, then along the A40 to Carmarthen.
Anything south is invariably lumpy and hard going for the first 50 miles or so from Aber, and for that reason the vast majority of our DIYs have gone West or North...
You may also like to get in touch with Dave Lewis about the Cambrian Connections Mesh, which would certainly give you the building blocks. Are you aiming to make a 200 from it?
AC