Pretty sure rclone uploads just fine without server dependencies, yeah. I never installed anything special on my home NAS and it happily accepts uploads with rclone.
That I can't really speak of. I know it does not reupload the same files at least (uses timestamps) but never really checked about only uploading file diffs.
Nothing offhand, but basically it can't know what's on the server without reading it all, and if it can't do that locally, it'll have to do it remotely. At that point, might as well re-upload the whole thing.
Its front page hints at this, but there must be details somewhere.
I think you’re misunderstanding something. There’s no need, and even no possibility, to have “rclone support” on the server, and also no need to “read it all”. rclone uses the features of whatever storage backend you’re using; if you back up to S3, it uses the content hashes, tags, and timestamps that it gets from bucket List requests, which is the same way that Restic works.
Borg does have the option to run both a client-side and a server-side process if you’re backing up to a remote server over SSH, but it’s entirely optional.
Not to make this an endless thread, but I have been wondering about what's the most rsync-friendly backup on-disk layout. I have found Borg to have less files and directories which I would naively think translates to less checks (and the files are not huge, too). I have tried Kopia and Bupstash as well but they both produce a lot of files and directories, much more than Borg. So I think Borg wins at this but I haven't checked Restic and the various Duplic[ati|icity|whatever-else] programs in a while (last I did at least a year ago).