I'm not very familiar with the area, but there might also be some freight companies around (FOC for short).
It seems that's the case in some GWR depots. Some people being in Talent Pool they applied for a Train Driver position, but over the time they started offering Depot Driver roles to them with an option to change to Mainline Driver in the future.
My advice is practise, practise, practise. And research. But also don't over do it, so you don't start panicking and overthinking. In the end of the day, they're just tests checking your natural ability for the job. I only used one of tools mentioned by you and I found it very helpful. There's also some tools available on this forum, for example Group Bourdon.
General approach is a X lines roster. Let's say, 50 lines. That's 50 drivers spread across 50 weeks. You start, let's say, on week 23 and you just follow it until 50, then start from 1 again. You can then plan your roster forever in the future - well until roster changes. There isn't a cut out example roster that works everywhere. It varies between depots and even inside the depot between different links. You work some number of days, then you're off for some number of days. That usually changes week to week. You could be working 4 on, 2 off, 5 on, 3 off, 7 on, 4 off. Or any other possible combination.
Shifts are obviously utterly unsocial, so you might be starting as early as 3 am and finishing as late as 2 am. Some depots do night shifts as well.
Good luck in achieving it!