Hugo Kaun's work exhibits a deeper, more post-Romantic intensity than other pieces of the same name. It is harmonically dense and requires a mature sense of structural phrasing.
Managing the voicing of thick chords while maintaining an expressive melodic arc is the primary technical challenge.
Comparable difficulty and character — good candidates for what to learn next.
Am Rhein (Op.90) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
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.
A typical performance of Am Rhein (Op.90) lasts about 6 minutes.
Am Rhein (Op.90) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include 'Ch'io mi scordi di te?' by Wolfgang Amadeus Mozart, Ballade by Edvard Grieg, Balada mexicana by Manuel Ponce — see the full similar-pieces list on this page.