So where are the Nottingham to Cardiff drivers? The service is still hourly as it was before and these seem to be taking the big brunt of the cancellations.
You're missing the point.
Birmingham to Cardiff was always 1 tph, but Birmingham to Nottingham was 2 tph. The same was the case on the Stansted route, with 2 tph to Leicester and 1 tph beyond. Both of these routes were reduced during the pandemic to 1tph throughout but have now been restored to their former frequencies.
That certain service are taking the brunt of cancellations should indicate to you where the problems lie. There is a big depot in the Midlands that has been under-complement for quite some considerable time due to various reasons. This has never been fully addressed and it is likely to take some years to deal. The core of the problem is that the company cannot keep up with the rate of people leaving, primarily due to retirement. Although it is still taking qualified drivers from elsewhere, this depot is still going to need a large influx of trainees to meet the demand. This hasn't been helped by the greater burden of the work diagrammed since the pandemic falling onto this depot while others have seen their jobs cut back.
The Covid pandemic has had a big impact on training, and not just for new starters. Route and traction training has been similarly affected due to the restrictions imposed on the number of people who can share a driving cab. In ideal circumstances it would take a driver roughly 12 months to pass out during which time they are classed as non-productive (in other words, they are unable to cover their own work). This is the case also for drivers who are route-learning or undergoing traction training. What this means in practice is that every driver undergoing training represents one additional job that requires cover. This is also the case where drivers are non-productive for other reasons not related to training.
Even while there was an agreement for rest-day working, even with the reduced service on some routes, XC was only just getting by. All spares were used and allocated together with rest-day volunteers, but there was still a daily need for additional people to come in and cover work. There has also been judicious use of cross-cover between depots and even reallocation of drivers on the day.
If a company is supposed to cut it's coat according to it's cloth, this is how much the coat can cover when all the elastic has been taken out. You're wrong to suggest that 50% of the drivers have disappeared off the books because you don't see what the situation has been within the company and how it has been covering all the work thus far. From a staffing perspective, XC is under-resourced due to the number of unfilled vacancies and number of non-productive drivers. Therefore, knowing the extent to which they have been reliant on the goodwill of their staff to come in and work on their days off, they have had to plan for cancellations and make them known in advance. Resourcing the service based on a known quantity is far better for everyone (including the passengers) than promising a full service at the start of each day and then making short-notice cancellations as the day goes on.
So you see, it's not as simple as you seem to think.