Travel from St. David's to Aberystwyth on a Sunday

What are the rest of your trip plans? Could you, for example, pick up a car in Cardiff and complete your entire trip by car? It would be by far the best way to see rural Wales. As you have found out, public transport is not that good.

Hi Helen, Thanks for your reply. We have done tons of travel in Europe and generally make it a point not to rent a car. We found it very stressful the one time we did it. However, I am thinking we may need to make an exception. Even just Cardiff to Aberystwyth by car might help--and car parks by the day don't seem too unreasonable--and perhaps some of our lodgings include parking. Still I'm hoping there is another solution out there!

You haven't made an error, just a big and very reasonable assumption that did not pay off-
The solution is to get a taxi for the 16 miles to Haverfordwest station, then the train at 1139 to Carmarthen, arriving at 1217, then the bus T1 from outside Carmarthen station at 1309, arriving at Aber at 1520
https://passenger-line-assets.s3.eu-west-1.amazonaws.com/transportforwales/FCYM/T1-timetable-20230326-0bb3ed58.pdf

This will cost you £18.50 for the train/bus combo.

Quite remarkably you can buy a through ticket at Haverfordwest for this journey (although it is not an advance fare, maybe buy ahead of time to save any confusion at the station on the day, on the TfW website).
I found this by chance expecting a rail ticket to send me via Cardiff and Shrewsbury and cost an arm and a leg.
The ticket shouldn't be hard to buy. I've just been able to make a dummy purchase on the machine at my local station in North West England, so a station in area should not struggle.

That will surely work out cheaper than a 3 day one way car rental and you can both enjoy the scenery.

I don’t anticipate you have any issues with parking. My best advice is get a good quantity of pound coins as there is no, or poor, mobile signal in a lot of places and so you can’t pay by phone, and very few car parks are set up for card payments. Pretty much everywhere you want to stay will either have parking or will be able to tell you where to park easily. If you have a car you can stay outside of town centres in quieter spots.

We had a rental car on our trips to Wales. When making lodging reservations we always ensure they have parking. St. David’s is very small while Aberystwyth is larger, but even it was easy to drive in. Between the two places we stopped at Pentre Ifan Burial Chamber and St. Brynac’s in Nevern to see the weeping yew.

The T1 bus has just started using electric buses which do one full return journey, before being recharged at Carmarthen.