1st time visiting Wales

Train Ticketing isn't a problem - it's finding hotels which is your problem on a Public Holiday weekend.

Just buy your Explore South Wales from the ticket clerk at Reading when you arrive, plus the cheapest possible ticket to Newport (South Wales) where the pass starts, then stay on the train at Newport.
It's important to stress it's the Welsh Newport, as there is also one in Essex.
There are almost always last minute Advance (train specific) tickets.
For instance at 1459 today there were still Advance tickets for the 1513 next train to Newport.
In South Wales your pass includes all buses- for instance the buses to St David's from Haverfordwest and Fishguard, also the St David's circular coastal.

Also the #349 Tenby to Haverfordwest hourly bus.

On the train you usually change at Swansea for the West but there are a few through trains to Carmarthen, and a summer Saturday through London to Tenby train.

Thank you for replying, but when you mentioned to buy a ticket to Newport/South Wales, do I also need to mention that final destination to the ticketing clerk as well? Will I take a train that will end somewhere else? I don't quite understand the return route of London via Tenby, can you help to clarify?

For the south Wales explorer, is Carmarthen a good base? Or would Swansea or Tenby be a better option? I just saw it's half term next week, so if I can't find the right accommodation, I'll go after the school holidays. As I'm travelling alone, I would like to get your thoughts as to where is best to stay for better connectivity on trains and buses.

If it was me I would stay in Carmarthen.

The reason being that Carmarthen is on all three West Wales rail branch lines, whereas Tenby is half way down one of the branches.

There is a good argument for Tenby, and it's a good base. I just think Carmarthen is a bit better. It is a bit of a tough call.

It sounds like you are living or staying in Reading for some time - rather than being someone having a short stay in Britain? That being so, I suggest that you do not make a trip to south Wales this weekend because after months of fine weather a storm is coming in from the Atlantic. Click this link and then the play arrow to see the expected track of the storm. Also a public holiday weekend so accommodation will be sold out or expensive.

Cardiff is only 1 hour 30 minutes from Reading and can easily be done as a day trip - thus saving accommodation costs - though to see more, at least an overnight stay would be preferable. Note that peak time train fares do not apply on the weekends and it can be done for £65.50 return by splitting the tickets at Swindon (you don’t leave the train) as Super off-peak return Reading to Swindon = 33.60 + off-peak return Swindon to Cardiff 31.90. Of course, specific train advance tickets could work out cheaper but you don’t know what the weather will be when booking well ahead. (Splitting the ticket lowers the price v a through ticket - sounds bonkers but that’s Britain’s railways for you). If you have a Railcard, you can reduce the prices by a third.

For Gower peninsula, Swansea would be your place to leave the train and take a bus run by Adventure down the peninsula. Rhossili at the SW end of the peninsula should be your number target - though the whole of the south Gower coast in particular is very scenic.

Carmarthen is a good transport hub but the town is rather miserable. I would make straight for Tenby and stay at least 2 nights. You may like to know that on summer Saturdays, GWR have through express trains to Tenby from Reading/London. One departs Reading at 9.13 to arrive Tenby at 13.16. (When boarding at Reading - be sure to be in the rear 5 coaches as they will split this train in Swansea).
Returning from Tenby - through GWR express trains leave Tenby at 10.38 and 15.38 to arrive Reading around 4 hours later.

From Tenby, you could take a bus (First Cymru 349 or 381 to Haverfordwest for another bus to St.David’s). If doing this, to get back to the railway for travel back east - head to Haverfordwest train station.

If you are using the Explore South Wales Pass, you do not have to tell the clerk your final destination and it is not worth buying if just going as far as Cardiff. If you are buying the pass and travelling further west, you would only need to your train tickets to cover the Reading to Newport section.

Hi James, this weekend weather & timing might not be great for sightseeing & walking, indeed I'll just go on another week. I'll look into a better plan for going for a weekend. Thank you for your suggestions.

Good idea - keep an eye on the forthcoming weather.

As you are in Reading, you may like to go to nearby Henley-on-Thames during the first week of July to see the regatta. You don’t have to pay anything as the footpath alongside the Thames is a public right of way. You will see plenty of posh people in various enclosures.https://www.hrr.co.uk/your-guide-to-henley-regatta/