I built a similar tool to what OP shared, mainly for debts instead of investments, and I completely agree with you. My app only uses iCloud to sync and it keeps all sensitive data on the user's phone. Another benefit I want to share about this approach is it means that apps built local-only will never have the risk of one day going offline and being inaccessible due to the company closing down or turning off the server.
Loving this so far, I've been working with it for a week now. I have a personal app, DownPay for iOS, that is built with SwiftUI that I want to bring to Android. I tried going the React Native route to build an android-only version but the context switching between SwiftUI, React Native, and then my day job made it challenging. I also tried Ionic and Ignite and wasn't successful with those either.
After testing the feasibility of other cross-platform frameworks, I landed on Skip. I LOVE that I don't have to break out of the "Swift" mental context, I just have to focus on writing an app in 1 language.
So before I commit fully I've been testing it out (building a demo app this week) and so far I am very impressed. The syntax to write platform-specific code (#if !SKIP #endif) is very easy to use once you get the hang of it.
It's amazing I don't have to learn Android to get something up and running at this speed with Skip. Hitting run in xcode and watching both emulators open feels like magic. I want to put this to the test so I plan to build a complete App with it from start to finish, ship it to both App Stores, and if all that goes smoothly I will proceed to migrate my main app using Skip.