iCloud for Windows not uploading photos to iCloud Photos

How do I trigger iCloud Photos to upload files from my PC. iCloud for Windows (v15.8.127.0), iCloud+ 2TB plan, 575GB used. Large photo library (~99,854 local files, ~58,620 already in iCloud). Uploads have failed twice at large scale ("failed to archive existing files and cannot start" error, resolved via uninstall/reinstall each time). Now testing with a clean, minimal 1,000-file batch of new local files — zero Send/upload activity observed after 30+ minutes, icloud.com/photos count unchanged, no error shown in the app, storage confirmed not full.