Thank you for the pointer to Leonie, always helpful. The post is a little old (2016) but I guess it still holds.
Basically what it says is that no, you can't have referenced photos in iCloud.
Then come the question to have it backwards and have other programs reference their photos from inside the Photos database, that would be managed from Photos's point of view, but referenced from theirs.
First I'm not sure I can do this, as currently Aperture is managing the whole and I am not sure I could keep all my current info while transferring it to Photos, (we're talking 30 000 pictures, tagged), ans still keep Aperture to manage my pics
Second, I don't think I can constraint Photos to put only compressed version to the cloud. Since I have 320 GB of pictures, that puts me in the 120 € / year plan, which is too much for the service.
Third this would also raise question to what do I do with the iPhone pictures that comes automatically in the Photos database from the cloud.
Fourth, I don't like the idea that something in the cloud could destroy my database. (for exemple inadvertently erasing pics on my iPhone)
I guess I have to keep going with my current workflow : having a scaled down version of my database duplicated for Photos, for consultation only.