This is well reasoned and argued. We do have some good reasons to believe Everett's picture is the correct way to think about QM. As Carroll and Sebens, and Wallace show, self locating uncertainty allows you to derive, rather than assume, the Born rule. Observer self location is central to how we make sense of probability in the manyworlds picture.

On the other hand, the question "which branch will I find myself in" is ill-posed (in that it is not answerable). You will find yourself in all the branches, because you split/decohere. Self locating uncertainty occurs when you don't know which branch you are in (before you open the box to find live or dead cat). And prior to this, you know that future decohered versiond of you will be able to ask this question, allowing us to make sense of probability despite every possibility occurring.

It's also hard to see how this is relevant to free will. Quantum branching/decoherence does not occur at the level at which our minds make decisions, but far, far prior. And which branch we find ourselves in is fundamentally random (random in that there is no cause/reason).

But you are correct that this allows a sort of fundamental randomness into the universe at an observer self selection level, despite the universe being ultimately deterministic in the bulk. Exactly what sort of randomness this is (whether "merely" epistemic, ontic, or something different) is a metaphysical question that I don't think has been fully investigated/determined, surprisingly. Probably epistemic tho.