Just mash it up. Even when K was just starting on food, I would mash them really well with a fork, and that was thin enough. Same for raw avocado and cooked sweet potato.
I just scrape the spoon on the banana and give it to DD that way. Sometimes I put some in a bowl and mash a little but DD does well with it being thicker/chunkier. I don't think you need to puree it completely but I guess it depends on how your LO does with food.
I put a little formula while I mash it up so that it is a little thinner, but DD is a bit younger than your LO so this maybe why she has problems with chunky banana bits.
Re: If I give DD bananas