Yes - the Cambrian Line is more scenic than the north Wales Coast Line - which is nothing special on the first leg out of Chester. Chester by the way, is well worth a stop if you can fit it in - though if you have Advance tickets - you would have to split them at Chester. You can see a British TV programme about the Cambrian Line here - though it does skip some things that I think should have been mentioned. https://visitcheshire.com/locations/chester
Note that when you reach Porthmadog that the Cambrian Line station is at the north end of the main street and should not be confused with the narrow gauge steam railways found at the south end of town.
The trains used on the Cambrian Line are known to have unreliable air con. A coach with closed windows indicates that the air con is working. New trains may be on this line by the time you arrive as they are currently making test runs.
You may also like to know that a narrow gauge steam railway links Caernarfon with Porthmadog though not sure that you would want to use that plus the long trip down to Cardiff all on the same day. https://www.festrail.co.uk
PS. When in Llandudno, you may wish to consider visiting Bodnant Gardens by using Bus 25. You can find the schedule here & select all the stops & the day of travel.
You can also use the same site -https://bustimes.org - to find other local buses - such as the 32 from Cardiff to the Museum of Welsh History at St.Fagans Castle - about 7 miles west of the city centre. (No bus on Sundays and note the last one back is about 3pm so best to go out on the first bus).
Also note that whilst in Cardiff, you may wish to visit the medieval castle at Caerphilly - about 8 miles N of the city. Buy return train tickets on the day and on arrival at Caerphilly Castle - show the tickets to get 2for1 entry.