A little surprised no one has mentioned OmniGroup's [0] applications.
Some of their applications (e.g. OmniFocus) support syncing online via an Omni account, but also offer support for syncing to a custom WebDAV server[1]. I run my own WebDAV server and point the macOS and iOS apps at it.
Some of their applications (e.g. OmniFocus) support syncing online via an Omni account, but also offer support for syncing to a custom WebDAV server[1]. I run my own WebDAV server and point the macOS and iOS apps at it.
[0]: https://www.omnigroup.com
[1]: https://support.omnigroup.com/documentation/omnifocus/mac/3....