I use an open source app called Loop Habit Tracker. All it does it pop up a persistent notification around the time that I need to do a thing (such as brushing my teeth, taking a shower, or taking my meds), and all it does is ask "Yes" or "No". No cloud, no account, no syncing with anything; entirely local. There's no accountability outside of a consistent line of checkmarks. Which can be disheartening if I missed a chunk. But it's genuinely helpful.