@Tomos: Sorry, but I thought Google's blocking this [API] was already quite clear from the Picasa blog:
clear from their blog as of March 26th 2018 (it may have been clear earlier, but I dont think so) -- either way, I, and lots of other people using Picasa this way didnt know about it.
There's nothing to stop you from using Picasa as an image management tool/database, and shipping up the photos to Google Photos, as required. (I had thought that was what you were intending doing.)
the nice thing about Picasa is you can create as many albums as you like:
Family photos are a good example -- using Picasa's face-recognition 'face-tags' I created albums for each family within the extended family; an album for my mother's family; an album for each brother and sister; for each aunt/uncle; etc. They're all subsets of the full collection of photos. AFAICS these albums are now useless (because I hadnt synced them yet).
I am unable to reproduce them in google photo without starting from scratch (it doesnt recognise Picasa's face-tags). I guess I'll just copy them from the Picasa album to a new local folder and offer them as zipped downloads -- not so accessible, especially to non-computer-literate people. Although...
I wonder if I upload an image twice to google photo -- to two different albums, will it recognise that it has the images already, and just 'tag' the copy it has already with the new album.