I was born in Merthyr and lived there till I was about 20 and often come back to visit my folks and mates. I live in New Zealand now but still miss lovely Merthyr, it’s seriously underrated! I was visiting home last month and couldn’t get over how friendly and warm everyone is. Oh and valley girls 🙂
There are some dodgy parts (you want to avoid the gurnos for starters!) but also some nice areas and like you say you don’t have to live in Merthyr itself, plenty of the adjacent valleys are nice too and very cheap housing compared to the rest of the country. You could also look at living on the edge of the national park and commuting in.
There’s awesome riding all over the valleys without needing to drive anywhere. The trail centres are all close (CwmCarn/Afan, Brechfa, Nant y Arian/Cwm Rhaeder are driveable), some great DH tracks (gethin/taff buggy/mountain ash/rheola and loads of shonky secret local stuff) and if BikePark Wales takes off I’m sure everyone will be changing their tune 🙂 There’s miles of XC stuff in the beacons and the colliery spoils in the valleys make for epic tip surfing on your bike.