I like your idea of penalties for departments that don't table their departmental plans in a timely manner. I'm all for that. But I think a more realistic and feasible approach to ensure that you as parliamentarians could better hold a government to account would be fixed dates for a budget, or a fixed window that would be early enough in the year that the main estimates would reflect budgetary expenditures and budget initiatives.
As well, the departmental plans would reflect new items that are in the budget for them. You collectively would have a good sense, when you study the main estimates, of how that reconciles with the budget, because budget items would, by and large, be found in the mains. Departmental plans would also reflect the government's priorities as stated in the budget.
With a budget tabled in late March, or really anywhere in March, or even April, it's not feasible for the departmental plans to reflect budget priorities, and it's not possible for the main estimates to paint an accurate picture of government spending. That's why I've been suggesting, for a while now, that a budget date sometime in February, preferably early in February, would facilitate your job and would also ensure that what is in the mains makes sense to you and reflects what's in the budget.