Problem lies in the fact that I have no Apple developer account, which is required to publish to the App Store. Accounts cost $200/year. Sheesh.
That sounds like a "you" problem, not a community problem.

Most people in signature campaigns make that every month, so donations would not be an issue if even one person had an iPhone. I think the common problem we all share is - are iPhone users intelligent enough to use such a tool?

What do you intend to do with the advertisements of the forum? If you are striping them out you would be taking revenue away from the forum. And if you are taking them out, why even bother keeping the functionality of this decade+ outdated forum? If Theymos has no problem with you taking away his personal advertising revenue, why not just import your data into a modern forum architecture? You don't need to enable posting or any other content creation (THAT would be crossing a copyright line I'm sure), but you could just use the advanced features of the software. Example:
https://elon.report/ With the recent version 5 major upgrade, anyone using something like that would never come back to this garbage. Most members here are great, but the serious ones leave for forums they can view on their phones.
Have you discussed the forum future with Theymos? He's finished his scam; the longer he stays in the public eye, the more attention he'll draw. His plans may include selling the forum (what happens to the 1,000 donated bitcoins?), selling the content for a ML model on bitcoin investing or shutting it down altogether. I would imagine now that he's out of his prime learning years he'll want to settle down somewhere quiet and watch Trump destroy the world. :/
Finally, here is what DeepSeek thinks about bitcointalk members using your phone app:
Crypto users are more likely to use PCs for complex and security-sensitive tasks, while still relying on mobile devices for convenience and quick access. Non-crypto users, on the other hand, overwhelmingly prefer mobile devices for most browsing activities, with PCs reserved for specific tasks. Crypto users' behavior reflects their need for security, functionality, and flexibility, setting them apart from the general population.