I see that your title says Breton Beacons instead of Brecon Beacons.
Now look at this map:>https://www.google.co.uk/maps/place/Llandeiloemail@example.com,-4.0757197,12.14z/data=!4m5!3m4!1s0x486e5454b5330a4b:0xd59a1e8d7c74a1d8!8m2!3d51.88469!4d-3.99143!5m1!1e1
It shows the way you will be heading to the BB from St.David’s. You will be on the A40 to Carmarthen and can continue on that road all the way to Brecon going via Llandeilo and Llandovery. However, you may wish to take the A48 for a short distance E of Carmarthen and then visit the National Botanical Garden of Wales? If so, on leaving this place, follow the B4310 NW west to the Towy Valley & turn R onto the B4300. Soon you will see a sign for Dryslwyn Castle and it is well worth the short detour to see this ruin beside the Towy. (Climb to the top for fab views). Then return to B4300 & go E. https://botanicgarden.wales
Just SE of Llandeilo is CARREG CENNEN CASTLE - a ruin set on top of a very high cliff. https://cadw.gov.wales/daysout/Carreg-cennen-castle/?lang=en The farm at the base offers food in a barn. If you visit this castle, be sure to go down the passageway into the cave underneath. (The approach road to this castle is narrow). Now, you could go on backroads via Gwynfe and head NE into the Brecon Beacons National Park. However, I would not advise this for non Brits as the roads are VERY narrow and you will have to drive backwards to a passing place if you meet anyone. It is also easy to get lost. So, probably best to head back to Llandeilo and join the A40 for your journey towards Brecon via Llandovery. If you are heading towards Merthyr, take the A4215 from Sennybridge/Deffynog.
You don't say exactly where you are staying. Merthyr is an old industrial town and was one of the first places in the world to produce iron. It is not worth a visit but if you are into industrial stuff, you may wish to visit a coal mine (free) at Big Pit, Blaenavon.
Also consider:>http://www.showcaves.co.uk/prices_opening.html If going to the caves, you may also like to consider visiting some waterfalls. Scwd Gwadys is about a 30 minute walk from the Angle Inn at Pontneddfechan. Other waterfalls can be reached by following the road towards Ystradfellte and parking where you see a farm track going off to the east. The Upper Club Gwyn Fall is an easy 15 minute walk. Others are more strenuous and longer from roads - click & drag the following map NE and they will show up.
The only narrow gauge steam railway in this area is found just N of Merthyr - be careful to follow signs as it is confusing:>