Currently it's impossible for mods to control FAPI's page system in creative inventory without using impl.

Here are some features this API should include:

• Allow getting and setting current page index (CreativeGuiExtensions impl)
• Getting the page count
• Getting the page associated with a specific tab (CreativeInventoryScreenMixin#fabric_getPage)
• Get a sorted list of tabs on a specific page
• Get/set the currently selected tab (vanilla accessor)