I can’t give you an answer, but what I can tell is that, unfortunately, being on iOS, you won’t have a lot of options when it comes to FOSS apps.

Apple charges developers $100/year to be able to publish on the app store, so that gives very little incentives for iOS developers to publish their app without any kind of monetization, whether via a subscription, ads, or mass data gathering.

I have a lot of respect for FOSS apps on iOS, because not only do devs offer an app FOSS, but they’re paying that $100/year out of their own pocket.

Joplin; it‘s actually more of a note-taking app with TODO functionality. If you want to sync across devices (there‘s a desktop app as well) you can select selfhosted options. Joplin is FOSS and doesn‘t collect data.

I went down that rabbit hole once and it’s really hard to find a foss app that is enjoyable to use and actively maintained. Condution (https://www.condution.com/) seems to be one of them but I never tried it as it looks a bit too much for what I personally need.

Not open source but e2e encrypted and privacy focused is lunatask (https://lunatask.app/). They have a free plan and you can get access to the iOS beta (the app is fully functional). I don’t used it anymore but I paid for Lunatask and it helped a lot making order in my brain and switching from bloated apps to a more minimal todo approach. It’s highly opinionated, but it implements more than one “method”.

Currently I’m using https://github.com/sodenn/2do-txt because I switched to the todo.txt method, which better fits my need for a more minimal approach to those things. The app has ios/macOS version (with tauri build replacing electron) and you can sync with whatever you want (i use syncthing).

Hope it helps, the todo landscape on ios is a nightmare.

Standard Notes is the best notes app on the market. It’s a cross platform and encrypted FOSS app which works great for to do lists.

Notesnook

I gave up and just use iOS’ native reminders app… which is actually quite good.

Why not use the stock reminders since you are already on iO? You can also use this app. https://www.condution.com/

Edit: I saw it’s already recommended by another user.