Plan your projects with the elegance of a sloth on a sunny day.
Vikunja, the fluffy, open-source, self-hostable to-do app.

As well as the self hosted app, a hosted service is available which supports the project.

OpenTodoList is a todo list and note taking application.

Organize todo lists, notes and images in libraries, which can be stored either completely local on the device you are working on (and hence ensure no information leaks out to untrusted third parties) or use the built in synchronization features which allows you to synchronize your libraries across devices. Currently, the following backend servers are supported for synchronization:

NextCloud
ownCloud

Generic WebDAV servers - note that due to limitations of some servers, the sync might not be working optimally or at all, this is a limitation of the server that the app cannot do anything about. Feel free to raise an issue in case you think your server is still capable to run a sync properly
Dropbox

In addition, a library is just a directory holding the items of your library as simple files - this allows you to use any kind of third party synchronization tool (like DropBox) to sync your information.