I suspect it really needs more training examples. The problem I found when I looked for images to use was that 60% were of dancers, and from past experience, it will end up trying to fit a dancer into every image you create. But of course, there are only a (small) finite number of Degas images that you can train with.

A possible solution may be to incorporate artificial images in the training data. So, create an initial LoRA with the original Degas images and generate 500 images. From those generated images, pick the ones that most resemble Degas. Add those to the training set and train again. Repeat until (hopefully) it learns the correct style.