It is not worth taking the Heart of Wales Line and just not viable for the route you wish to take. The trains on this line are old with no air con.
On the Conwy to Abergavenny route the following places are worth hopping off for and then taking a later train:> Chester, Shrewsbury, Ludlow. When going east from Conwy, the train goes along a not particularly attractive section of the Welsh coast.
From Chester to Wrexham it is relatively flat farming country. Once south of Wrexham the scenery is more interesting. You pass through the Shropshire hills once south of Shrewsbury as you approach Ludlow. Then rolling farmlands down to Hereford. Between Hereford & Abergavenny you will see the foothills of the Black Mountains on your R and then Skirrid Mountain on your L.
Hereford might be worth a look but it is a good 16 minute walk to the centre/cathedral from the station. Ludlow does not look up to much on arrival. Walk towards the Tesco supermarket (where perhaps they will look after your baggage). Go to the street on the west side & turn L. After the Feathers Hotel (VERY OLD) - take next R for the centre & castle. Best view of castle is from the other side of the river on W side.
If you pre-book an Advance specific train ticket - note that you can’t hop off and back on a later train(s).
It might be worth you getting this pass which would be valid as far south as Shrewsbury.https://tfw.wales/ways-to-travel/rail/ticket-types/rovers-and-rangers/north-mid-wales-pass
Other passes/rovers exist - it depends on what your intentions are and the number of days. Here is the full list of passes/rovers for Transport for Wales:>https://tfw.wales/ways-to-travel/rail/ticket-types/rovers-and-rangers
Compare that with just buying tickets.