Trip planning!

I suggest you head to somewhere just over an hour away by train from Bath - Cardiff, the capital city of Wales.
Did you know that 4 castles can be found in & around the city?

https://www.cardiffcastle.com/the-castle/ is found in the centre and dates from Roman times. In the middle is a Norman keep (1087) but much of the castle dates from the 1800’s when it was owned by the Marquess of Bute.

CAERPHILLY CASTLE is a massive medieval fortress just a 20 minute local train ride north from Cardiff. The town of Caerphilly is a dump in an ex mining area but the castle is everything that you would expect of an 1100’s fortress complete with moat. If you are visiting Caerphilly, you might like to break the journey at Lisvane station and walk for 5 minutes across the car park and into CEFN-ONN-PARK (free). In spring it is very colourful with azaleas, camellias, rhododendrons and bluebells all under the canopy of a temperate rain forest. https://www.outdoorcardiff.com/parks/parc-cefn-onn/
https://cadw.gov.wales/visit/places-to-visit/caerphilly-castle

CASTELL COCH stands on a hillside in north Cardiff. If you have a car - it is M4 junction 32 and you could easily visit this castle and Caerphilly Castle in one go - or taxi between the 2. If visiting by bus from Cardiff - you need Stagecoach 132 from Greyfriars Road and alight at Tongwynlais (about 25 minutes out). The walk from the bus stop to castle is about 17 minutes uphill.
https://cadw.gov.wales/visit/places-to-visit/castell-coch

ST.FAGANS CASTLE & MUSEUM OF WELSH LIFE is just to the west of the city. The site is vast and needs at least half a day. You have grounds with buildings of significance brought from all over Wales and re-erected. The Castle is really a 1500’s Manor House. You will also find gardens near the castle. You will also find indoor galleries explaining about life in Wales through the ages - handy if it starts to rain. https://museum.wales/stfagans/about/

LLANDAFF CATHEDRAL is about 2 miles NW of Cardiff Castle.https://www.llandaffcathedral.org.uk

If visiting Cardiff, I suggest you visit CARDIFF BAY - which is just over 1 mile south of the city centre.
https://www.visitwales.com/destinations/south-wales/cardiff/top-things-see-and-do-cardiff-bay

https://cardiffmuseum.com is a small museum about Cardiff in the centre of the city. THE NATIONAL MUSEUM OF WALES is found just N of the shopping area next to the City Hall. It has extensive art galleries.

If visiting Cardiff, note that the city holds major events - especially in the Principality Stadium. This can result in hotels charging massive rates to cash in on all the fans and transport systems are stretched. (Taylor Swift is now booked for 18 June 2024).
Best to avoid such events if of no interest.
https://www.visitcardiff.com

Bristol would be the obvious choice. Just a few minutes by train and there is also an excellent bike path between the two.

I might be inclined to base yourselves in Bristol as there is a lot to see, you have been to Bath several times and transport options are better from Bristol. You can for instance easily get to South Wales for James' ideas.

You could come to Bristol where I live! It’s 10 minutes on the train from Bath.

A much bigger city than Bath with lots going on. You could do day trips by train to different places such as Cardiff, Stroud, Frome and Bruton or even down to the south coast eg Weymouth.

Wonderful ideas all! Really appreciate the help.

Tintern Abbey (ruins) is supposed to be a nice ride from Bristol.