It's meant to be 2 years before you can apply for a transfer between depots but some in the north have been known to move a lot sooner than that if there's a vacancy going and no one else wants it. I know down south guards have been able to make the jump from valleys to mainline within a couple of years but I I've heard anything from 5 to 10 years as a driver.
On the guards side the Valleys depot is bigger than the mainline, so I would assume the same is true for Drivers, so that does explain part of the wait. It's also worth mentioning that within the mainline depot there are numerous links for drivers; even if you did make the transfer, don't expect to be going to Milford Haven or Chester on Class 67s any time soon. You'd be spending a lot of time doing the VOG, Maesteg and Ebbw Vale lines which aren't much different from the Valleys really!
I suspect Cardiff is fairly unique on the railway in being a half way point between 1 depot and 2. They share management and booking on points, but are treated as two different depots for rostering purposes; other than shunts around Canton and Cardiff Central, they don't share any work. They also have slightly different T&Cs, for example Mainline is a 5 day week, Valleys are 4.
The only similar situation I can think of in Britain is XC for Senior Conductors vs TMs at Birmingham and Bristol, but even then the drivers at least do mixed work, and XC recruit directly for both sides whereas anyone starting in Cardiff for ATW is put straight on the valleys.