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 (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 (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 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.
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 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.
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