How do I balance my carbs and protein if I simply want to maintain muscle bulk?

There is a fine line between (a) getting enough carbohydrate to fuel your exercise requirements and maintain muscle mass, and (b) getting too much carbs and causing body fat accumulation. The key for you is to find this balance and maintain it for the long term. This you should find easy.

The other issue is that you may not need the extra large amounts of protein needed to retain new muscle mass, but keeping your protein consumption at about 1.9 grams of protein per kilo of body weight should enable you to easily retain the muscle mass you have worked so hard for. Start your carbohydrate intake at approximately 1.9g to 2g per kg body weight and see how you go. If you begin to accumulate body fat, reduce it a little, alternatively, increase it if you find you begin losing your muscle mass.