Did Dickinson experience much development as a poet as she grew older, or did her work largely remain the same?

I don't think that any poet's work can stay the same as they battle life's challenges. They can have a similar voice and tone but people and poetry change. This was especially so for Emily. When Emily returned from boarding school, she was very active socially, and was considered well-liked and attractive. In her late twenties, though, she suddenly cut herself all from all society, never leaving her family’s home, and started ferociously writing poetry. Although there is a long-standing myth that the catalyst for this was her falling in love with a man who rejected her, it is more likely that it was a combination of several factors. Emily’s mother fell ill with an undiagnosed illness, and from then until her death in 1882, she was essentially bedridden, and Emily and Lavinia had to devote a great deal of time to caring for her. This was especially taxing on Emily, who found all domestic chores stifling, and who was not very close to her mother. Finally, between 1851 and 1854, as many as thirty-three young acquaintances of Emily’s died, including her good friend and cousin, Emily Lavinia Norcross.

Emily began to dress only in white, and would see no one but her family, meeting visitors only through screens or behind doors. She wrote prolifically, writing almost 1800 poems in her lifetime, but her genius was never recognized in her lifetime. If you pay attention to the chronology of her poetry you can see the changes both dark and introspective through her work.