Customize your FPP times right away, starting with the days where you want the harder to get FPPs. The later in your stay you are booking, the easier it is to get the “harder” FPPs (kind of like ADRs).
Use the Optimizer to help you determine which FPPs will benefit you the most each day - see Making Optimized Personalized Touring Plans with FastPass+ - #48 by Loras for some tips on how to do this.