Staffing issues aside, while it’s not strictly true (at my TOC anyway) that the reason for a reduced service is to allow more maintenance to take place, it’s the opposite, we have less exams of a weekend, and less maintenance requests of a weekend, Sunday’s especially, don’t forget why would there more maintenance staff of a Sunday than a week
However, on a Sunday you have a complex job of balancing units, and keeping units spare due to exam mileage, which is one of the reasons the Sunday offering isn’t as good (staffing issues, aside of course).
More services = more units, you’d have to expect that maintenance of a evening and day during the week would have to greatly increase due to increased unit mileage.
It can be frustrating as instead of running a train service, you are in effect running a maintenance plan.