Recently I got the 100-400L II after I considered the 400 5.6L, the first having better IQ (despite being a zoom), IS (4 stops), and the photos are very crisp. I shot the N America nebula with full aperture and it is sharp to the edges.
It weight slighly more (1560g) but is shorter than the 400mm prime.
Check Gumtree one of A$2400:
https://www.gumtree.com.au/s-ad/theb...usm/1192762348