So what wound up working for me was replacing [band] with [line_renderer]. I assume the problem was because Band was no longer a valid target for bokeh's hover tool as bigreddot mentioned in their comment.

So that section of the code now looks like this:

    # Adding hover tool for confidence band
    hover_band = HoverTool(renderers=[line_renderer], tooltips=[
        ("Speed", "@x"),
        ("Lower Bound", "@lower"),
        ("Upper Bound", "@upper")
    ])
    p.add_tools(hover_band)