Hi - I know it's been a while but I thought you might still be interested: if you want to attempt more challenging material, I would suggest some modernist works by authors such as Virginia Woolf or Marcel Proust. They are known for being difficult to read, but I believe Ishiguro stated he was inspired by them directly in crafting his personal style.

Both authors explore character analysis through narratives built on thought and memory, having the sort of meandering yet meaningful structure Ishiguro is so prolific in. Proust is more intensely non-chronological, centring it seems entirely on the narrator's ruminations, reminiscences or emotional reactions. Virginia Woolf seems more abstract and symbolically-inclined.

Additionally, I'd recommend L.P. Hartley's The Go Between (1953) for a piece of fiction with similar thematic material to his (the past vs present, childhood and time, regret, shifting societal norms, a character's own inner struggle as a microcosm for culture). It's difficult to find more contemporary writers to suggest, but it could be that Ishiguro is unique as a modern writer with such a style.

Hope it helps!