Mozart's K. 104, likely a set of dances or shorter character pieces, would showcase his characteristic clarity and melodic grace. These works, though less prominent, are valuable for understanding his compositional evolution.
Developing a refined touch and a clear understanding of classical phrasing are key to interpreting these works.
20 Minutes (K.104) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
20 Minutes (K.104) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.