I don’t know the actual route, but the difficulty probably depends on how you divide up the 56 km. This guided hike makes it a 5-day/4 night hike, and gives the elevations gains and losses for each day, plus the hiking time (most are around 6.5 hours).
https://www.mountain-sports-zillertal.com/fileadmin/user_upload/Wichtige_Informationen-Sommer/EN/Peter_Habeler_Runde.pdf
Some days have 1100 meters, more or less, of elevation gain (or loss), others have less. 1100 meters of gain would rate the day’s hike as “strenuous” in the system used by my local alpine club (Seattle Mountaineers). But there do not appear to be any technical sections.
Looks like this is a website that rates it as “black” but I don’t know what that means in terms of difficulty in the Austrian Alps. A different website I found gave it a 5 out of 7 for difficulty.
https://www.tyrol.com/things-to-do/sports/hiking/hiking-tours/a-peter-habeler-route