@thedancingbear What about things like big museums, the British Museum, the V&A, the National Gallery? There just isn't anything like that outside of London. What about when the OP has a five year old and wants to take them to see the dinosaurs at the Natural History Museum? That's going to be a hell of a lot cheaper living in a London suburb than if she lives in South Wales.
Most people who are regularly going to gigs at the Camden Roundhouse are either twentysomething hipsters who live in Hackney or at least people who work in Central London and would go straight from work. Wherever in London they live, they're more likely to be able to go to that gig than someone who lives in South Wales, unless the band is touring and stops off in Cardiff.
Nobody is saying there is no culture in Cardiff. There is loads of culture in Cardiff. But if you want things like big museums and galleries, or West End shows, even gigs (and yes, I know there are a lot of musical events in Cardiff), it's going to be easier to get to them if you live in Croydon. So suggesting that Croydon is a shithole with no culture is a little bit disingenuous, given how close it is to London. Obviously if you want to be doing these things all the time, you'll need to live close to good transport links.
But all of this is a bit irrelevant to the OP's predicament really, given that her main concerns seem to be finding a decent job and living somewhere that is reasonably diverse and a good place to bring up children. Even someone who is going to gigs three times a week before they have kids is probably going to be more concerned about mother and baby groups, kid friendly cafés and baby swimming classes once they've had them. The OP will want to live somewhere she can do those things, and where she can make friends. Cardiff or Bristol might well fit the bill, if she can find a job in the post-coronavirus apocalypse. Abercarn certainly won't.