Have you considered the species of banana you're using? Your post prompted me to see what else in the banana family was suitable for wine, and found that the Cavendish variety (most popular in the US), was created in the 50s when the original Chiquita variety was hit with Panama virus. The Cavendish is not as sweet as the original Chiquita and has more starch. Apparently not many varieties are available in the US anymore, but the Red Banana has become increasingly available in high end grocery stores - these apparently are sweeter than the yellow variety and less starchy - but more expensive. Here's a basic description:
Red bananas, also known as Red Dacca bananas in Australia, are a variety of banana with reddish-purple skin. They are smaller and plumper than the common Cavendish banana. When ripe, raw red bananas have a flesh that is cream to light pink in color. They are also softer and sweeter than the yellow Cavendish varieties, with a slight raspberry flavor.
Your post has prompted me to start coming up with a recipe and I am going to see if I can find any red bananas locally. I have some leftover fig concentrate, and think it might make for another interesting desert style wine - Red Banana, Fig and Agave is my thought!