I finally succeeded in putting the Wordpress media files on S3 and served by cdn, on a test site. This should not have stumped me this much, there's a plugin for it to help and I managed to configure it on Pleroma and PeerTube without the help of a plugin. Maybe these have better instructions...

One of the issues was that the instructions of W3 Total Cache said to give minimal S3 access permissions to the access key but doesn't say what that is, so I did GetObject, PutObject and DeleteObject but that doesn't work. Blog posts acknowledge this and say to just give full permissions. Fortunately AWS had a list and it also needs ListBucket, GetObjectAcl and PutObjectAcl. Even if Acl's are disabled. Just ignore that the test upload doesn't work. Also ignore people who say that the S3 bucket needs to be open to the public.
#Wordpress #W3TotalCache