[citruspi]

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.

[0]: https://www.omnigroup.com

[1]: https://support.omnigroup.com/documentation/omnifocus/mac/3....