Cryptomator

Cryptomator solves one problem properly: your cloud provider can read your files, and you would rather it could not. You point it at a folder inside Dropbox or iCloud Drive, it creates a vault there, and unlocking that vault mounts it as a normal drive you can work in. Everything written to it is encrypted per file, filenames included, before it ever syncs. Dropbox sees a directory of meaningless names.

It is the standard recommendation partly because it is free and open source, and partly because it is boring in the right way — no account, no key escrow, audited, and the format is documented so your files are not hostage to the app.

The honest limits are on mobile and on sync behaviour. The desktop app costs nothing, but the iOS app is a €29.99 purchase and Android another €29.99, and Skymatic says outright that a licence for one platform does not carry to another. Vaults also sync more slowly than plain folders, because editing a large file rewrites encrypted chunks.

If what you actually want is an encrypted cloud service with sharing and recovery built in, this is the wrong shape — Cryptomator has no concept of sharing a vault with a colleague unless you both have the password, and team key management is a separate paid product.