I lived in London for four years before moving to Cardiff. Ive been here 3 years now (though I'm actually from Northampton originally - I moved to Cardiff because my fiance is welsh and has a steady job here).
There are pros and cons for both areas. My experience was that probably 8 out of 10 jobs I got while living in london weren't actually IN london, so I was having to rent my room out while getting digs in other areas of the country. This still applies in Cardiff - but in that regard it really doesn't matter where you're based, you'll always have to travel to where your next job is. The auditions were all in London - but I have no problems getting the train from Cardiff to London for auditions. I do have a relative I can stay with overnight in London, which is massively helpful, and I've also stayed there while doing acting jobs in london that have lasted a month or two. If there is no-one you can stay with for short periods in London then you might find it tricky.
The cost of living in Cardiff is much lower, which equals less time having to do fill-in jobs to pay rent, which means more time to put into finding your next acting job. Also, the biggest thing is that there's a HELL of a lot less competition for acting jobs in Cardiff. There are still a lot of amazing trained actors, but nowhere near the volume that are in London. Plus all the National Theatre Wales castings specify that you can only apply if you are welsh, trained in Wales or have a strong connection to Wales, which rules out a lot of competition. (Of course, there's nothing to stop you applying for these castings while still living in london, because you're Welsh.) If you are welsh-speaking, even better - this is quite often specified as desirable on casting breakdowns.
To sum up, there's a fantastic creative community in Cardiff - helped along by National Theatre Wales and the BBC as well as many smaller organisations. My experience so far has been extremely positive. It's a smaller pond and I feel much more creatively nurtured here somehow - less overwhelmed. And if you're still willing to travel around the country as jobs dictate, then it really doesn't matter where you're based.
Hope this is helpful - let me know if any questions! : )
Ps. to directly answer your question - I would say there are definitely less castings in Cardiff than in London, but your chances of actually being cast are higher.
I'd say my biggest source of jobs for the past couple of years has been CCP and Singers Pro, although I have has to travel for a fair few of them. I assume you've got your CCP filter set up to alert you to jobs outside of london, so that you do actually see the south wales castings?
Another good source is the Actors group on this website, which I'm guessing you already get alerts from?
Other than that it's pretty much just keeping my ear to the ground, going to NTW events, various film networking events, meeting people. A great thing here is that I feel like I know, or at least know of, the majority of theatre and film up-and-commers in the area. I don't think that would be possible in London! So there's more opportunity for building a good reputation and working with people who know you.
Hope that helps.