For some reason I am not seeing the videos.
But what I suspect is at the zoom you are at trying to get three lanes, B/W during the day is hard because of contrast issues.
You could probably bump up the shutter even faster - try 1/10,000 and see if that adds some clarity for the daytime B/W.
But if you can read the plates in color at 3 wide, then I would go with color during the day and B/W at night.
Performance in B/W during day is way different than performance at night with B/W. Contrast, brightness, etc. is way different for day settings B/W than at night B/W.
If you plan to run color during the day, then the only reason to do B/W during the day right now is to get the focus number. Everything else is irrelevant in the settings. You save adjusting the settings for at nighttime.
I could read my plates during the day in B/W but they really pop when I went to color during the day. The daytime versus nightime B/W of plate reading are literally night and day. During the day in B/W - you see a lot of stuff in the image at various shades of grey. At night, the plate pops off the screen as all you pretty much see is the plate and the IR really illuminates the plate.