Characters that you don't really care about are one of quite a few fairly common problems with scifi. Others would include (but are not limited to) things like:
>excessive forced neologisms
>simply awful attempts at humour that is cringeworthy at best and literally reddit-tier at worst
>a completely linear evolution of contemporary morality and social values (usually just modern day liberalism taken to the nth degree)
>something I like to call the "WOAH, you still use PAPER?!" effect, you know what I mean
and so on...

The problem with Blindsights characters isn't that they're so posthuman (or approaching posthuman) that we cant relate, it's that he doesn't write them well enough to make us care for them all that much. Watts is not a writer. He is a scientist who decided to write scifi books. This has its ups and down, with one of the most notable downs being that he cant write much more than 1 dimensional characters. His attempts at padding the novel with backstory of Siri are weak at best and a slog at worst. Characters that are unrelatable to us can still be cared about immensely. One example is in Flowers for Angernon. Charlie is unrelatable to most people in both his intellectual peak and trough, yet we care about him hugely, often to the point of tears. Unrelatable characters CAN be ones we care about, but it takes someone who can write beautifully, not just well and interestingly.

I hope Watts asks for some artistic help from his editor when finishing the Blindsight series. His technical side is superb and his plots are well paced and punchy, exciting even. An editor who can put him in contact with someone who is a natural writer will make for a fantastic novel to round out the series.