Karl Weigl was a notable Austrian composer whose late-Romantic style remains largely overlooked. This work demonstrates sophisticated contrapuntal handling and dense harmonic textures.
Mastering the dense chordal textures and clear voice leading in the varied treatments of the theme is the main hurdle.
Comparable difficulty and character — good candidates for what to learn next.
28 Variationen über ein achttaktiges Thema (Op.15) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8.
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 28 Variationen über ein achttaktiges Thema (Op.15) lasts about 15 minutes.
28 Variationen über ein achttaktiges Thema (Op.15) 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 'Fuga sopra il Magnificat by Johann Sebastian Bach, Chromatic Fantasia and Fugue in D minor by Johann Sebastian Bach, Partita No. 2 in C minor by Johann Sebastian Bach — see the full similar-pieces list on this page.