FWIW, you don’t need to pay the $99 until you’re ready to release on the app the store. In other words, you can develop and test locally for free. That should give you an opportunity to see if you’re truly interested in going all the way with iOS.

I’m going to sound very negative here and it isn’t because I don’t like open source software. I use it and contribute to it. The problem with OSS apps is that they get cloned by people who don’t care about the license and repackaged with predatory subscriptions or with malware. In the case of malware these lowlifes go out on sites like Fiver and offer to pay unsuspecting developers to distribute the app. If the app is downloaded even once, that developer now faces a lifetime ban from distributing Android apps. I suspect similar things happen on the App Store. It’s just more visible in the Android forums I follow than it is in the iOS ones.

I have seen stories of oss apps being cloned and then Apple mistakenly not letting the original dev upload updates because the app has been flagged in their system as being a spam app or built from a template. This is usually correctable with enough email to support.

My recommendation is to keep your app closed source on both platforms. If you want to contribute to the communities, release a library or contribute to one. If you want to show How to write an app, make something minimal and trivial like a todo list. You can also create a blog.