I can agree with you that the X3 is only worth it north of Pontypool!
Yeah S2 is definitely a good route! And I'm doing the T4 soon, but that's a topic for another thread.
So I wonder what the point of TrawsCymru is then, if the X3 being part of TC would make no difference?
I think you'll enjoy the T4. I haven't done it for a long time, and I fail to remember what the vehicle type was now (possibly Mercedes Citaro) but I remember one trip to Builth Wells was with Stagecoach's 25103. This was back in the late 2000s, so a long time ago!
You will definitely like the Merthyr to Brecon route, going through the Brecon Beacons National Park. Hopefully you'll get nice weather for it, unlike the wet day I had. Some incredible scenery on that route, and some long stretches of fast running are possible
Speaking of scenery, I'm not sure exactly which way the T6 Brecon to Swansea goes, but it definitely goes to the village of Sennybridge as I saw it there on an epic Gran Fondo ride last year. Assuming it does the sensible and direct road from there to Swansea, with no doubt little diversions to serve villages just off the main road, then you will absolutely love that for scenery too. Another route that takes on the beautiful Brecon Beacons National Park

The T6 is on my list to do as a priority, but I've been to/from Swansea many times so I'm well versed with the scenery, which is why I can comment on it
Anyway, if you ever want advice on bus travel in this part of the world then I'll happily try and advise. I have a number of routes I want to do in Gloucestershire for the first time and, in a couple of cases, a revisit after nearly 15 years. That's all lined up for future exploring! I really should get back on topic, before I come up with more ideas for days out!
TrawsCymru, as someone else pointed out, is designed more to connect with the Welsh railway network, and the X3 doesn't really do that as such. Yes it connects some places to the railway, but only really in the same vein as many other bus routes around the country.
The old X4 from Hereford to Cardiff via Abergavenny and Merthyr Tydfil was much more belonging to the TrawsCymru brand as that connected up places that are much more circuitous by rail. I did that route in 2004 or 2005 I think, it was way too long ago to be sure. The X4 and indeed X3 back then used Volvo B10M coaches, the height of luxury compared to noisy and rattling Enviro 200s now, but obviously not even slightly convenient for anyone not able-bodied.
There certainly used to be a Brecon to Llandovery bus route too, another one I did many years ago on an Optare Solo, which would have been useful as a TrawsCymru route. I don't believe it operates at all now.
Another route that would belong better than the X3 to the TrawsCymru brand would be the 461 from Kington, Herefordshire, to Llandrindod Wells. There's another route I can recommend for scenery, by the way, and its main route from Kington to Hereford is also a nice ride. It used to be my regular commute until 2017, so I know it well!
Point being that there are routes much better suited to the TrawsCymru brand, and in my opinion if one of the routes out of Hereford was to be given the designation it would probably be the 36 from Hereford to Monmouth. With a suitable route onwards to Newport ideally too. Although I would say Monmouth has good connections to Abergavenny (Route 68 with Adventure Travel, another good one to do), Chepstow and Newport as it is by bus, and I'm not really seeing the value of putting TrawsCymru branding etc on any of those routes. If it would suddenly lead to a big increase in usage, then fair enough but given the Chepstow bus uses bread van-sized buses anyway I don't think it's too likely that it is all that well used.