Does "Download and Keep Original" require manual sync of each photo?
To downloads all photos, Download and keep originals will retrieve all images from the cloud and save them to your phone.  Optimize storage will only download when photos are viewed and/or are needed by an application.  

It may require you to plug your phone in, connect it to Wi-Fi and wait for a bit for all of the photos to download, as you said there are quite a few.  It may help to have Photos open beforehand too.

Source: Experience &amp; Apple Support documentation.