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

This should work well for what you are asking,

Dickinson's poems generally fall into three distinct periods, the works in each period having certain general characters in common.

Pre-1861. These are often conventional and sentimental in nature.[132] Thomas H. Johnson, who later published The Poems of Emily Dickinson, was able to date only five of Dickinson's poems before 1858.[133] Two of these are mock valentines done in an ornate and humorous style, and two others are conventional lyrics, one of which is about missing her brother Austin. The fifth poem, which begins "I have a Bird in spring", conveys her grief over the feared loss of friendship and was sent to her friend Sue Gilbert.[133]

1861–1865. This was her most creative period—these poems are more vigorous and emotional. Johnson estimated that she composed 86 poems in 1861, 366 in 1862, 141 in 1863, and 174 in 1864. He also believed that this is when she fully developed her themes of life and death.[134]

Post-1866. It is estimated that two-thirds of the entire body of her poetry was written before this year.