Ugly solution, but If you use UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "None") it will use the name of the class as a category and will sort it at the top. Note that you can’t use subcategories with None. Also if you want to search the category in the BP graph, you can search for |None.
Also a slightly “better” solution for some may be to use UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Default"), as the Default category is close to the top in instances (but at the bottom in class defaults). But with this at least you can use subcategories which will be usable in the BP graph editor.