Taskwarrior on Desktop and Taskwarrior + Termux on Android is the most flexible, private, open and lightweight TODO manager I've ever used. No eye candy whatsoever, just pure power. I've been using the combo for a few years now, give it a look.
How do you sync between devices?
Also check https://github.com/coddingtonbear/inthe.am
You can use their hosted version or host your own.