Seems perfectly readable to me, and I haven't used Java professionally in over a decade. What specifically do you find problematic?

The ~12 levels of indentation and 5 layers of nesting are doing my head in personally but maybe my brainspace is too small.

I think this perspective explain the issue well: you pretty much cannot unit test this (unless you consider the whole function a unit). It becomes very unclear what each lambda expression is about, the deeper you get.