The Tregaron Mountain road mentioned by Richard is very narrow - you may have to reverse to a passing place.
When in Llanfairfechan, I would suggest that you drive just a short distance to visit the Aber Falls - which is about a 45 minute walk up a valley from the parking place.http://www.eryri-npa.gov.uk/visiting/walking/accessible-walks/Aber-Falls-Path
Also be sure to visit Conwy (castle) and Bodnant Gardens from Llanfairfechan. Also consider the Snowdon Mountain Railway and Caernarfon Castle. If you want beaches - head over to Anglesey. Llandudno is worth a look if you have time.
LLANWYRYFON. The drive here will take you through spectacular country. I would take the A470 to Betws Y Coed. If you want the fastest route - stick with the A470 all the way to Dolgellau. However, even more scenic is this route from Betws-Y-Coed :> A5 to Capel Curig then A4086/A498 to Beddgelert & A4085 to visit PORTMEIRION - an Italian style village you pay to enter. Then take A496 coast road to visit HARLECH CASTLE before heading south to Barmouth and up the Mawddach estuary to Dolgellau. Then it will be A487 to Aberystwyth though you may like to detour via B4405 to Tywyn and on to the ABERDOVEY.
I have never heard of LLANWYRYFON. I think that a day on the VALE OF RHEIDOL RAILWAY from this location would be enjoyable combined with a look at Aberystwyth.
On your transfer day down to FISHGUARD, I would call in at these places:> ABERAERON, NEW QUAY, maybe LLANGRANNOG - lack of parking means using a mini bus link from village edge - look on street view. Proceeding SW, PENTRE IFAN BURIAL CHAMBER near Newport is worth a look.
From your FISHGUARD base, I would go to STRUMBLE HEAD and then down the coast to WHITESANDS BAY & ST. DAVID’S. SOLVA is also worth a look. On day 2, drive to the south Pembrokeshire coast & visit the seaside town of TENBY. Then MANORBIER, BOSHERSTON - park by church and walk past the lily pools to the coast - turn E and you reach BARAFUNDLE BAY. On return - visit PEMBROKE (castle).
FISHGUARD > CRICKHOWELL :> maybe NATIONAL BOTANIC GARDEN OF WALES (just E of Carmarthen) > CARREG CENNEN CASTLE (in the hills E of Llandeilo) > BRECON BEACONS NATIONAL PARK MOUNTAIN CENTRE (which can be reached via a backroad if you are good with maps) > CRICKHOWELL.
CRICKHOWELL is surrounded by lovely countryside. If there weather is bad, consider going down the BIG PIT COALMINE at Blaenavon (free). Llangors is OK but also consider climbing to the top of the BRECON BEACONS - but beware of the weather. 3 soldiers died last year on the mountain due to the heat! The valley going from Talybont SW past the reservoir is very pretty. Where the road crosses a stone bridge prior to a steep hill, park the car and follow the track for about 20 minutes & pass through a metal gate to BLAEN-Y-GLYN FALLS.
CRICKHOWELL > NEWTOWN. From Builth Wells, the A470 is more scenic than the A483 to Newtown. At Rhayader, consider taking a circuit of the ELAN VALLEY RESERVOIRS.
NEWTOWN = visit POWIS CASTLE (near Welshpool).
Google place mentioned for more info. How you did all those countries in 2 weeks I’ll never know.