Upload specific photo album to icloud?

iCloud Photo Library is all or nothing. You either upload the full library or no photos at all.

If you want only selected albums in iCloud create two Photos Libraries, one to keep only on your Mac, and one that will sync with iCloud.

See: Use multiple libraries

Or use shared albums to send photos to your other devices. You can set up 100 albums to share to other devices. You can share them simply to yourself and do not need to share with others: See: iCloud: iCloud Photo Sharing FAQ

iCloud Photo Sharing doe not require iCloud Photo library - those are two separate and distinct services with no interdependency

You also can log into iCloud.com via Safari and upload selected photos

LN

Shared albums are using a separate service of iCloud from iCloud Photo Library. Shared albums do not count against your free iCloud storage.That is why some limits apply:

See this link: https://support.apple.com/en-us/HT202299

attention, shared albums don't save photos in original size.

Seems that setting up photo sharing requires the ability to sync my entire photo library in the Cloud, so back to square one...

Yes, exactly. Use shared albums only to view photos on other devices, but do not delete the originals from the library on the Mac.