> I was thinking about whether to return an error. If we can’t find a UTF-8 start byte in the nearby 4 bytes, it’s unclear what to return. I thought maybe we could ignore this problem.

This highly depends on the use case, and your stated use case doesn't seem to need any sort of error.

> Also, I want to avoid copying large strings.

Go strings are not copied in that way; they are implemented like immutable slices [1].

[1] https://research.swtch.com/godata