the night is young, the computer is hot

Some of them are understandable enough, but I imagine it can change from town to town, and when you live on area borders it gets a bit confuddling what it’d be classified as.

I remember a discussion on what dialect Epynt residents would have used (the citizens of Epynt were made to move out of their Welsh speaking community for those who don’t know) and it seemed very up in the air. It seems like certain dialects are more under threat than others and specific/mixed dialects and historical usage aren’t widely available knowledge.

I'd say your best resource would be Cymraeg, Cymrâg, Cymrêg - Cyflwyno'r Tafodieithoedd. It's several decades old now, but it still contains invaluable information about the different dialects spoken across Wales. The book is in Welsh, although the maps should be fairly easy to decipher even if you don't speak the language!

Interestingly, it doesn't use labels such as Powyseg for the dialects, but instead categorises them geographically: North West, North East, West Midlands, East Midlands, South West and South East Wales. That's how I personally referred to the different dialects growing up; I didn't even encounter the more 'official' names until much later in life, and I reckon that's fairly common among fluent Welsh speakers.

I'd say the only dialect with a particularly clear boundary is Cofi, which is specifically associated with Caernarfon. As soon as you leave the town, the speech shifts back towards the broader Gwynedd dialect - but there might be other examples that I'm less familiar with!

Welsh dialects are otherwise quite fluid, with fuzzy boundaries shaped by the way people move around the country, especially as more and more people leave their rural communities to find work and education in the bigger towns and cities (which is what I did). For example, you'll often hear children in Cardiff who are learning Welsh as a second language using words and expressions picked up from their teachers, many of whom come from North or West Wales.

Another very interesting resource is Speak For Yersel, a really cool digital project that explores the differences in English across Wales. Even though it's specifically an English-language project, a lot of Welsh words are a firm part of the Welsh English dialect by now, so you can use the heat maps to view key words, such as the variants for 'grandmother' (nain up north, mam-gu down south):