Answer: In general, peppers develop heat which can also taste bitter, I think, in their flavoring when they are stressed, especially if they are water stressed during the ripening period. They also tend to become sweeter when they are fully ripe. The best flavor actually appears when they turn red! Based on your description it sounds like you are basically taking good care of your plants, but you might try increasing the water a bit and wait to be sure the peppers are truly ripe. Also, remove the inner white ribs from the peppers because they are definitely the most bitter part.
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