Absolute best among ToDo apps is https://tasks.org/.

It's FOSS, and has great UX.

Maybe you know todoist.com, tasks.org are very similar and comes free and open.

They have backup and can be synced with https://f-droid.org/packages/org.decsync.cc/ + https://syncthing.net/

You don't need payed, closed apps full of spyware and ads. The FOSS apps are great and they works.