Taking a train from Moreton-in-Marsh to Conwy … but where to stop?

There are multiple options from Moreton in Marsh to Conwy.
Via Birmingham, routeing Oxford or Worcester (the latter is my personal preference) or via Hereford then the Welsh Marches line.
Yes Conwy is small, but I don't think anyone is saying it's not worth it. What I don't get is why that is the only place on the coast the forum thinks of staying at.
That area is well worth it.
Personally, and this is decades of family history, I would stay in Llandudno. Absolutely visit Conwy, but apart from the fact that Rick's buses stay in Conwy I'm not sure why it has total primacy here.
I could see staying in Hereford, from whence you could easily visit the Brecon Beacons. Likewise I could see Shrewsbury for a couple of nights.
If you change trains at Shrewsbury you could go round the scenic Cambrian Coast, and stop for a few nights at Dolgellau, Tywyn, Porthmadog or elsewhere, then bus to Bangor for the train to Holyhead.
There are huge tranches of North Wales this forum doesn't explore but are fantastic for scenery, walking and the lesser known steam trains of North Wales.
You could go to Chester for a night. Then take the Traws Cymru bus down through Corwen to Bala for a couple of nights in either.
Even to Dolgellau. From Dolgellau you can do Harlech Castle among other things.
Especially from Corwen you can easily visit Llangollen.
There is a two hourly bus from Corwen to Bangor through the heart of Snowdonia to Bangor for the trains.
That would be a refreshing change from the usual identikit itineraries on here.

I get that Rick 's best of tours don't have the time to explore this part of North Wales, but that is unfortunate.

Does the 3 weeks after the Cotswold walk include time to be spent in Ireland?

Look at this rail route map. The most obvious route to me would be to go from MIM by direct train to Hereford where you can change for another direct train that will take you to Conwy/ Llandudno Junction. Now, you will notice that leg 1 of this goes through Worcester so it sounds like you will be stopping off at that small city. (I was not impressed by Worcester on the one time I visited).

Hereford is the next small city that you would come to and have to change trains. Like Worcester, Hereford has a cathedral and some old buildings. Not sure that I would want to make it more than a pit stop though it does have buses going W to Hay-on-Wye and Brecon. Trains also take about an hour to reach Cardiff - the Welsh capital.

Proceeding north from Hereford on The Marches Line, you will now be on Transport for Wales Trains. Ludlow is an historic town with a castle - best viewed from across the river on the west side. Ludlow is certainly worth a look but not sure that I would want to stay more than a night.

Shrewsbury is the next town going north and has a number of historic buildings and several train lines converge on the town. One of these is the Cambrian line going west to the coast and this line is very scenic. The first stop W of Shrewsbury is Welshpool where you will find Powis Castle - which is a bit of a trek from the station. So, staying over in Shrewsbury, you could consider that as a day trip.

You could consider going west on the Cambrian line all the way to the west coast and maybe stay in Barmouth or Porthmadog. Of those 2, Porthmadog is the better placed for travel into the Snowdonia National Park but I would not rave about the town.

The other alternative is to go N from Shrewsbury for a stay in Chester from where you could do a day trip to Liverpool. Chester is well worth seeing.

Finally, Conwy: small and attractive with a big medieval castle but also a stones throw from the lovely Victorian seaside town of Llandudno. Day trip possible from either to Bodnant Gardens - highly recommended. You could also take the Conwy valley line to Betws-Y-Coed or even the old slate mining town of Blaenau Ffestiniog from where a narrow gauge steam railway goes to Porthmadog. www.festrail.co.uk

https://www.visitwales.com = Welsh Government tourism site.

Hereford not worth visiting? It has the Mappa Mundi in the cathedral for goodness sake, as well as an early second edition of the Magna Carta -https://www.themappamundi.co.uk

Thanks so much for all the opinions and suggestions. MUCH appreciated. Clearly I have more research and work to do. (I wish we could go for a longer period of time: we did a 10 week trip once and I was sold on longer trips. Sadly our budget is not sold on that, though.) I’m glad I have a good amount of time to work on our itinerary.

If you happen to end up in Shrewsbury (it’s gorgeous!)
Csons does the best English breakfast you’re likely to encounter. https://www.csons-shrewsbury.co.uk/
Absolutely delicious. It also has a branch of Beefy Boys (an incredibly good mini chain)