What can I know with 100% certainty? Sorry, I had not noticed your question. Let me try to answer it here. You asked: "So, would it be fair to say that in your view, any biological organism making choices has free will if it's not completely deterministic?"
Can neurological processes be semi-deterministic? I don't think so. As far as I know, neurological processes are completely deterministic and therefore, we do not have any free will.
My definition of free will is a will that is free from determinants and constraints. I clearly don't have free will because my will is both determined and constrained by my genes, environments, nutrients, and experiences. I clearly have a determined and constrained will instead of a free will.
Of course, it is possible that I am an immortal soul who is experiencing the illusion of having a body and being on Earth. It's possible that I don't actually have a brain and body and cells and genes and environments and nutrients. It's possible that I only have the illusory experiences of being embodied. These are interesting ideas but they are totally untestable.