There are two very broad varieties of Welsh - we call them ‘North’ and ‘South’, but it’s actually much more complicated, because there are areas that will say some things using ‘northern’ constructions whilst also using ‘southern’ constructions for other things, and then there are areas that have their own little quirks that aren’t really covered by any courses. It’s often a case that towns only a few miles apart can have totally different accents and/or dialects, so it’s almost impossible for any course or book to cover them all.
Most TV companies producing programmes will try and get a variety of people from different parts of Wales to take part, so you will often hear a multitude of accents on S4C. But unless you know where the contributor is from, it may not be clear which dialect they are speaking.
There are podcasts, YouTube content, and S4C catch-up in Welsh online, so it’s possible to get a wide variety, so you will find (probably most likely on YouTube) things focusing on Welsh phonetics and pronunciation, but I don’t usually watch those so can’t recommend anything - perhaps someone else will jump in and say what they’ve found.
Cwmni Da have a YouTube page which has lots of short clips from their archive of programmes and there are many different accents to be heard in those, so those would be good for more listening practice. Here’s the link to that https://www.youtube.com/user/cwmnidatv
The biggest festival on Welsh culture is the National Eisteddfod. There will be loads of TV and radio coverage and a wide range of accents to be heard. It doesn’t really focus on language differences though - in general, to Welsh speakers, Welsh is Welsh and while they know there are different dialects, it really doesn’t bother them which dialect anyone uses, even when they come across dialect words they’ve never heard before!
Most general ‘Learn Welsh’ courses will look at grammar (SSiW is different - we concentrate on speaking, as you’ll have noticed!), and I think more are now available via zoom than there used to be. Again, perhaps others will jump in and say which they’ve attended. As for grammar books, you can’t go wrong with Gareth King’s ones. Lots of threads on here about those, but here’s a recent thread Gareth King's Grammar and Workbooks - #98 by M2017
Hope that helps a bit. 