I use and love Kopia for all my backups: local, LAN, and cloud.

Kopia creates snapshots of the files and directories you designate, then encrypts these snapshots before they leave your computer, and finally uploads these encrypted snapshots to cloud/network/local storage called a repository. Snapshots are maintained as a set of historical point-in-time records based on policies that you define.

Kopia uses content-addressable storage for snapshots, which has many benefits:

Each snapshot is always incremental. This means that all data is uploaded once to the repository based on file content, and a file is only re-uploaded to the repository if the file is modified. Kopia uses file splitting based on rolling hash, which allows efficient handling of changes to very large files: any file that gets modified is efficiently snapshotted by only uploading the changed parts and not the entire file.

Multiple copies of the same file will be stored once. This is known as deduplication and saves you a lot of storage space (i.e., saves you money).

After moving or renaming even large files, Kopia can recognize that they have the same content and won’t need to upload them again.

Multiple users or computers can share the same repository: if different users have the same files, the files are uploaded only once as Kopia deduplicates content across the entire repository.

There's a ton of other great features but that's most relevant to what you asked.

Borg backup has append only

No, I'd actually be interested in that myself. I currently just rsync to another server.

I've used rclone with backblaze B2 very successfully. rclone is easy to configure and can encrypt everything locally before uploading, and B2 is dirt cheap and has retention policies so I can easily manage (per storage pool) how long deleted/changed files should be retained. works well.

also once you get something set up. make sure to test run a restore! a backup solution is only good if you make sure it works :)

Restic can do append-only when you use their rest server (easily deployed in a docker container)