I was there last year and very Vereeeeee hot.
First off there did seem to be a lot to have a go at, from various walking trips around I saw loads official and and unofficial trails.
Second unless you like doing extreme stuff on long stem hardtail xc bikes DONT Bother trying to hire a bike. I tried to hire a full sus but I was nt german and my name was nt Hans. Unless you want to pootle about on the valley floor of course. The English guy in the Cube centre was Very condescending.
Also as I am British?/Scottish the brakes on the hire were in Ero mode Ie back to front.
After all that I would defo go biking in the Ziller Valley again but I will be taking my own bike or at the very least sorting out a suitable full sus. 😆