“I am good enough” can totally have a different meaning. A lot of the times it is used for psychological contexts. Some examples are: “Do not compare yourself with others, you are good enough.”, “ You are already good enough, don’t waste time proving yourself worthy.”, etc. For telling someone that you’re full, and you don’t want more food or drinks, you can say: “I have had enough, thank you.”, “I have had plenty!.”, “No, I better not, thanks.”, “I am totally satisfied, thank you so much.”. Jane: Would you like any more? Amy: Oh, no thank you, I’ve had plenty! I hope it helps!

You can just say “ I had enough “ or “ I’m full” .. the other answer is uncommon..

No. "I'm good enough" means something completely different. It is only used as a form of self-affirmation. As Kevin commented, you can say "I'm good" in response to an offer, but with "enough" it would be odd.

You can also say "I'm good,"--at least in the USA. This is very informal and would be used when a friend is offering you something more to eat or drink.

Yes yes