Mashing bananas for Muffins: By hand or with blender

Since it sounds like they are basically a dried out batter at this point, I would turn to implementations that rely on those kind of food-stuffs;

• With some further baking and possibly additional brown sugar and butter or apple-sauce, I would use them for a crumble topping. If they are coffee and cinnamon an apple-cardamom cobbler would benefit nicely.
• The crumbles could be incorporated into a biscuit batter recipe
• In all likelihood, any kind of recipe that relies on a granola could probably be referenced for further ideas.
• You could also use the crumble as a stand-in for oat based no-bakes. The no-bakes might feel mealy to the tooth, so not entirely eliminating the oat might work best.

If you wanted to repurpose, you might try thinking of it as a material to be suspended. With an ingredient like well-blended coconut butter (not to be confused with coconut oil or milk or cream) you could create a base with which to make a kind of post-raw bar cookie (basically press and chill the coconut butter in a pan on top of wax/parchment paper, pulse the muffin bits with some dates or brown rice syrup, and once the butter sets press the bits/dates mixture into the top and drizzle with some ganache or whatever).

If the muffins have a slightly chemical flavour, it could be that there is not enough acid in the recipe to react with all the baking soda. Bananas do contain malic acid and citric acid but you could try adding some lemon juice to be sure. You could also play around with the proportions of baking powder to baking soda. Try 1 1/2 tsp baking powder and just 1/2 tsp baking soda.

I also notice that the muffin in your picture looks slightly anaemic compared to your ideal muffin. Get an oven thermometer if you don't already have one and check that the middle tray of the oven is reaching the correct temperature.