Macsen is a very welcome sight - I'm delighted that it's still in use (or back in use?) after 1500 or so years. And Bethan delights me too, though I'd prefer Betsan. Is the th in Bethan voiced (dh) or unvoiced (th)?
Bethan is just English-style Beth with the Welsh -an ending. I get a kick out of Macsen too for the same reason. It is a recent revival, afaik - can only find one Victorian instance, & they seem to start cropping up in the 1990s.