What’s a good Microsoft To Do alternative that syncs between Linux and my phone?

If you don’t need a native desktop app, the web version of Microsoft To Do through Outlook is probably the simplest option. You can use it from a browser on Ubuntu while keeping your existing lists and phone sync.

There are plenty of alternatives, so it’s worth comparing a few based on the features you actually use. Amazing Marvin is very customizable and supports Linux, although it requires a subscription. Cfait was also mentioned as a possible option, but it may take some experimentation to see whether it fits your workflow.

Super Productivity is worth checking out. It has a Linux version and is designed for tasks, projects, and time tracking, so it can be a good step up from a basic checklist.

Mindwtr can work if you’re comfortable syncing your data between devices with Syncthing. It’s a more hands-on setup, but it avoids relying entirely on a hosted service.

TickTick is another popular cross-platform choice, especially if you like reminders and widgets. Just verify that the Linux experience meets your needs, since its desktop support may differ from Windows and mobile.