South -v- North SSiW is something that I’ve thought about quite a bit. I chose the Northern dialect for the simple reason that it’s personally easier for me to get to North Wales when I’m in the UK. But I have also gone over all the Southern material. I eventually found that it made more sense for me to just listen to the Southern challenges, because actually repeating the material (certainly when it differed from the North) led to more confusion than benefit. The problem seemed to disappear when I did the Southern ones as listening exercises only.
But it’s not really the most efficient way to gain familiarity with both dialects.
Would it not be possible to prepare a special challenge or two for exposure to the other dialect? It would only need to cover a few key structures and common words that aren’t obvious to learners and may not be easy to pick up. I was thinking of efo/gyda, eisiau/moyn, angen/eisiau, ydy/eu, o/fe, the “so fe” negative construction, and maybe one or two others, that’s all. The rest will come during normal exposure to the language.
I have noticed when listening to Radio Cymru that I can pick out these words/constructions with ease, whereas I don’t think that would have been the case had I not done the Southern material. I have also run into conversations with fellow SSiW learners, often at the same proficiency level as myself, who are unfamiliar with these very common and therefore essential Northern words.
I think the listening exercises would have pretty much that effect… 
Hi, Baruch! You welcomed me on my first Nia Hangout almost a year ago, and (given some breaks taken) I’m through the 6mws course (though not all its tasks, even yet, quite) and still going… I think I have to accept ‘somewhat haphazard’ as my learning style. It is sustainable.
I swapped from SSiW North to South and back to complete Level 2 in North only. Started Level 3 in North now. Like @Tricia I’m somewhat Canolbarthist (+ in my case, despiccably Lloegrist).
@Aran’s, @KateM ’s, and @Macky’s comments about school Welsh, have been striking home, as has your own recommendation to just use @Iestyn’s version henceforth more passively as listening material. So level 2 revision/Old Course material will include some South for me.
Having come originally& briefly only to the more mishmashy Duolingo before SSiW, I think Aran’s point of view might usefully feature somewhere in recruitment/induction material aimed at the likes of me as new beginners (ignorant of almost all things Welsh except pretty superficial awarenesses).
@neilrowlands recent link to BBC Bitesize missed out School Welsh at Primary Foundation for WSOL, his link taking us first straight to TGAU/GCSE. I’m finding comprehensible input in the Primary Level for listening, in the very first videos available, with Gog/De differences to spot among what complements SSiW Old Course & Vocab lessons well. School Welsh may not be real, but a lorra lorra children are (I suppose) well versed and expecting it. The PE teacher in the first video here seems to have a cold or a mild speech impediment/particularity. Wn i ddim/ I dunno… is that an expression from across Wales?.. it’s all great practice. Bethan here seems to tend towards Gog… https://www.bbc.com/bitesize/articles/zsjnk2p
Biggest point for me has been what Aran has said about how learners from outside can unwittingly undermine the self-confidence of authentic mother-tongue speakers to use what comes naturally to them as their language variety from a specific locality and/or family experience.
Just as an aside (I had to check that one), although I initially learnt De Cymru (Southern Welsh), and speak predominantly with South Walians, I must say that I have to watch myself, to steer away from using Northern/Gogledd pronunciation. Especially the ô sound.
Having said that, I am the same with English, arsking people from Barth about their grars.