Not that there's a lack of Lemmy apps. I want more mbin apps for sure, especially Eternity would be great as that has a good compact mode.

I think there is another one that can do both lemmy and mbin but it's only for iPhones. I don't remember the name of the the app. Just remembering being mentioned somewhere.

Yes, this community has a quite wide audience

Not sure I understand your question, but Interstellar is built with Flutter, and currently supports Android and Linux (x86_64) via Google Play, F-Droid, and Flathub. And yes, it's both Lemmy and Mbin.

I've looked into it before, and Apple developer accounts cost $100 every year, which I am not willing to pay out of pocket. If someone decided they wanted to donate that amount every year, then I would probably reconsider. I also don't have any Apple hardware, so I would need additional help in that regard; I could emulate macOS (which is a pain though), but I would need someone to actually test on a real iPhone.

I've been out of the loop for a while, but it started as a frontend for kbin before there was an API or mbin (as Ernest was still working at it) and the roadmap was aiming at Android first, later iOS. You might be in luck... but it could take a while.