Could Not Be More Grateful Meaning?

It could mean that the little appreciation you've expressed is all you can summon, or that you're so filled with gratitude that you feel like you'd float if you were any more thankful. In your context, you are expressing sincere gratitude with the phrase, especially if the rest of what you communicate says likewise.

Can't be grateful enough meaning?

Meaning/Usage: To be very thankful. Explanation: This is commonly used when you feel that simply saying thank you is not enough. If someone did something very special for you, then you are expressing that no words can repay what they did.

Could not have been more meaning?

"Couldn't be more" is a phrase you can use to express that it is not possible to be or have more of something. For example: "I couldn't be more satisfied with the way the project turned out." This means you are so satisfied with the project that you could not be more satisfied.