Fastrack tickets are timed - there are only certain amounts per time slot so they urge people to stick to their times to ensure your queue times are a minimum.
As for what you should do, personally id wait and see what its like on the day, theres no price difference between buying online and on the day so id play it by ear, if you get there and its packed then you can head to either Depth Charge or Inferno Fastrack units and purchasing the package / rides you want.
It maybe worth arranging your trip to fall into a "Summer Nights" event, tickets are either £15 or £5 for annual pass holders and all the major coasters are open between 7PM and 10PM, last year queues were at a minimum so it was easy to get multiple rides on all the coasters
https://www.thorpepark.com/events-experiences/events/summer-nights/