I tried to buy Express two years ago and it was sold out for opening weekend. I had 2 options though, upgrade my ROF to ROF+Ex or buy an Unlimited express pass. I did this in the early morning.
For the after 11 express, I've also been able to purchase at guest services for multiple days in the future, although his was 2018 or 2019. Anyway, the sooner you try in person the better.