If you've completed all the quests and are at 550/600, then for now, you cannot go any higher.

Quests take time and are still being worked upon in the background.

Uh, I saw this same question asked in mid-2022. We are heading into mid-2023 currently. I feel like what makes the game move, are the quests. Is there an update out about future quests that I missed, or has nothing been announced yet? This game is super cute, but if it's going to be another year or so until I can do anything else to progress, there's no point in playing really.

This game is run by a team of two and they have overcome a lot of challenges in the past year. 

While the story and quests are an important part of Horse Fable they aren't the only thing. There is still plenty to do with breeding training and showing horses.