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.