I have been to St David's today by bus.
At Haverfordwest (train from Bridgend) I asked for a £6.70 Day Explorer ticket. However the Richards Brothers website is out of date, and this no longer exists.
Instead I was sold a £7 Traws Cymru ticket. On the Traws website it says this is not valid on the T11 to St Davids. After a whole flurry of texts and phone calls by two different bus drivers this was established to be incorrect, and the ticket machines accepted the QR code.
Thus £7 (used to be £11) now gets you from Carmarthen or Haverfordwest or St David's to Bangor, Chester or Wrexham- a total bargain.
You would probably run out of hours in the day if you tried it, but worth knowing.
However, there is always a but, the loss of the Day Explorer means that to use the seasonal Strumble Shuttle or the Puffin Shuttle buses from St Davids you now need a West Wales day ticket costing £9 as they are not TC buses.