If you really want to optimize and share code across platforms, you should really go for C++. Make your core logic in C++ and write bridges for Kotlin and Swif

If you really want to optimize and share code across platforms, you should really go for C++. Make your core logic in C++ and write bridges for Kotlin and Swift, where only the UI logic is involved. C++ has the better ecosystem so far and the very modern standard (C++17) let you write robust code. There are also so many tools that help you catch errors and typos, plus thousands of libraries available.