That's a nice write-up. I'm also a student of Van Gogh letters and I also like to extract his technical timelines. Recently I've made a Wiki page with extracts from his letters and sketches about his Drawing Learning Journey: https://pardall.xyz/wiki/art/van-gogh/van-gogh-learning-journey
I liked your article because you elaborated very nicely on top of his letters.
By the way, I'm also a polymath (on top of being an autistic savant), so it's a pleasure to have discovered your articles.