"learning/mastering both Go and Swift is the best decision you can make" is a very bold claim.
In my area we use Go for some systems but found it scales badly from an engineering point of view to larger more complicated systems, compared to languages like Java and Scala. The JVM has much more mature tooling and libraries too.
For mobile applications it will be a while until Swift is stable on Android, so some of the cross platform languages would be a good choice. React Native, Unity, Unreal for example.
In my area we use Go for some systems but found it scales badly from an engineering point of view to larger more complicated systems, compared to languages like Java and Scala. The JVM has much more mature tooling and libraries too.
For mobile applications it will be a while until Swift is stable on Android, so some of the cross platform languages would be a good choice. React Native, Unity, Unreal for example.