My Photo Stream still uploads pictures after iCloud fills up? Please correct me if I'm wrong:

It's a good idea to turn on My Photo Stream in iOS or Mac if your iCloud is filling up because, if it fills up, your last 30 days of pictures will still upload even if your iCloud has been full.  So, if you lose/break your device, at least those last pictures should be in iCloud.
Apple covers this in the FAQ on Photo Stream and iCloud photos. It’s towards the bottom :


https://support.apple.com/en-us/HT201317



  Does My Photo Stream use my iCloud storage?
  
  No. The photos you upload to My Photo Stream don't count against your iCloud storage.