The October 2016 issue of Modern Railways has an article on the Conwy Valley Line. Interestingly, in the context of your post, it apparently has been given an increased promotional budget and, apparently, a new line logo.
That's good. The 'no growth' franchise in hindsight has left problems this end of the franchise. There again, it's a lose - lose situation as if more growth had been planned, there may not have been the capacity to match it.
ATW/WG don't go enough to promote railway lines in my opinion. If you go onto GWR's twitter page, website or even onboard their trains, you see posters and pictures advertising the Tarka line, St Ives line, Looe line etc and the branding (i.e. Logos) for other lines that aren't marketed as much is still strong, like the Heart of Wessex line. A lot of people probably don't even know the Conwy Valley line exists!
The problem is that trains on the route are too infrequent to do much with, and Sunday services are very poor. They could market walks between stations, or something like the Tarka ale trail but, realistically, you'd need double the amount of trains to make it worthwhile - And I believe there is capacity for this on the line with trains passing at North Llanrwst?
As for the coast portion of the Cambrian. It is largely seasonal. Most trains have two coaches all year around but they do get full in the summer months with tourists and hiking clubs. Many school kids also use them to/from Harlech or Tywyn. One thing that didn't make sense was when the evening Barmouth terminator was extended to Pwllheli, yet missed out most of the stations between Llanaber to Porthmadog, though this seems to have been rectified now. The Cambrian Coast is probably one of my favourite lines in the country due to scenery and being run by refurbished 158s - but the branding is weak. You can see on the old station signs, i.e. the ones at Pensarn that Barmouth Bridge was used as a logo at some point, but I don't even know if there is one anymore. 1tp2h is decent given the context, though there is the infrastructure to run 1tph I believe, passing at Tywyn, Barmouth, Harlech and Porthmadog. Not saying I would though, apart from maybe high summer.