Bach's "371 Harmonized Chorales and 69 Chorale Melodies" represent a monumental pedagogical work that codifies his mastery of hymn harmonization. This collection is indispensable for understanding Baroque part-writing and tonal harmony.
The primary challenge lies in understanding the intricate voice leading and harmonic progressions within each chorale harmonization.
371 Harmonized Chorales and 69 Chorale Melodies is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
371 Harmonized Chorales and 69 Chorale Melodies is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.