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.

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.

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.