Quite difficult to set out how the work can be done better when nobody knows either for a weekend, a year or a lifecycle what work is actually needed.
I will maintain that Christmas Week and one weekend a year is probably enough for 12 route miles and 15 days a year sounds like rather more than what is needed.
If you ever read a route's Periodic or Weekly Operating Notice and even in a fairly normal week you will see how much is being done, yet this is normally just for minor preventative maintenance.
When you say "I will maintain" what engineering experience do you base that on? You have then said "sounds like", so could you maybe appreciate that some of the technical expertise - that you seem so quick to shoot down as being inflexible - actually understand and have assessed what is required and then pass then onto operational planners to organise this for months in advance.
Evidently there are alternatives to look at but if it will end up causing the timetable to be non-compliant, a lot more hassle, and no the time penalty is so extreme then obviously it will not be progressed. Just because currently things are the way they are does not mean that a) various other options have not be looked into and these large blocks are the "least worst option and b) that it is fixed for ever and will not change.
As an aside GTR's bus operation at Three Bridges is fairly slick now with quite a lot of additional, knowledgeable staff assisting people on and off buses in a timely manner with standby buses available should there be a significant gap in coverage due to traffic. I think one issue is that the perception of "railway replacement" normally conjures fairly negative images and this has been reinforced in culture/media but if actually thought about in a meaningful way, it does not
need to be a miserable experience for passengers.