iCloud fails to sync all photos dropped into iCloud Photos folder on Windows 10

Background: My wife and I have used iCloud for years for photos from our iOS devices, not problem syncing from iOS to iCloud and to my Windows system. However, prior to converting to all cloud storage for our photo library, we had a PC with years of photos. I have copied those photos from the old drive to my current Win10 system.

I have been attempting to have iCloud Photo sync all these old photos into the cloud for a couple days now. It appears that *some* of the photos sync - and I can see them on iCloud (website) and my iOS devices (iPhone and iPad), but some do not. And checking the status of those 'missing' files shows that they do not have a status indicator, and the toolbar tip says "Excluded (not synced)".

I have been unable to complete moving my old photo library to iCloud despite having plenty of additional storage in iCloud. (I'm showing 118GB available) About 44GB of photos are failing to sync at this point.

Of course, the usual Windows restart as first line of defense fails to have any more photos uploaded from the folder.

This screenshot shows that folders 2001, 2002, 2003, and 2004 are synced (and freed from my Windows folder -- as I want them to live in iCloud and not on my PC); however, the remaining folders 2005, 2006, 2007, 2008, 2009, 2010, 2011, and 2017 have no status. Folders 2012, 2013, 2014, 2015, and 2016 appears to be synced - but if I dive into the structure below, a few random photos fail to sync. (see below)

Here you can see that IMG_7558 has been skipped over for no explainable reason. This strange behavior continues throughout the folder structure with no pattern I can recognize. I also can say that iCloud Photo is not making any further attempts to sync additional photos as the status shows 'up-to-date'. (see below) And no network activity is being recorded from iCloud's processes.

In addition, Apple Support Docs would have me 'turn off and on' iCloud; however, a very large warning pops up about deleting all the photos from the local device (Win10PC) -- which would mean I would have to *recopy* from the old PC's drive, again. Not something I'm really looking forward to repeating if at all possible.