Keep in the mind the roads and narrow and winding with a few surprises in the rural areas, so driving from the north to south may take a lot more time than you had planned. For recommendations on the south side, I stayed in Brecon which is a nice small town with good shopping locations, access to transport, restaurants and had good accommodations.

I don’t think there anywhere that’s ideally central without driving quite a ways. You can try and narrow it down based on what you want to see in Wales. If you like visiting castles, I’d suggest lodging in Snowdonia to cover the northern side. This also gives you easy access to the Lleyn peninsula which has beautiful views.