Firstly, everybody varies as to how many carbs they can eat, and the type of carbs makes a big difference as you have found out with Weetabix, most cereals are quite high in carbs and quickly turned into sugars in the body. They tend to be High Glycemic Index foods, and many contain extra sugar.
What you need are Low GI foods that are low or medium carbohydrate. The list is a long one so I will let you do a bit of research, but stuff like berries, almond flour porridge, eggs, Greek yogurt, Burgen linseed and soy toasted bread and so on are the sort of breakfast foods to eat.
So, eating low GI carbs will let you eat more of them, and almost all foods contain some.
I eat about 100 grams of carb a day, but this is mainly low GI carbs, Vegetables, some fruit, cheese. vegetable protien
(I am a vegetarian) pulses plus what's already been mentioned and other foods.
You just have to test 2 hours after eating and see how a food affects you, if it's OK then you can eat that and you don't have to test again because you know how it will affect you.
Some people go as low as 30 grams of carb a day, some can manage 150 grams or more it just depends on so many things.
Exercise makes a big difference, I can eat a lot more carbs if I do a lot of exercise, if I just sit and do nothing then I have to be careful.
H