For a full year request in advance, the admin work due to changes will be too much. This is referring to changes of leaves within the organization as well as due to changes of the individual.
Actually, in your email, the main problem is not being addressed on, and that is the accumulation of the days off and public holidays. I don’t see that below.
For the annual leave, a minimum requirement to get this one submitted/approved can be done with a time frame of 6 weeks, but no more than a quarter (3months).