To make future proof of your application, I highly recommend using SwiftUI. We are developing an application from scratch using SwiftUI for the last 5 months. SwiftUI still not fully matured, there are many shortcomings. But to create complex design declarative UI is way better than the traditional way to develop apps. Also, you can create separate reusable components which are called views in SwiftUI, and can create composable user interfaces easily.

Also, over 75% of user switched to iOS 13, so unless you need to support backward compatibility SwiftUI is highly preferable.

For existing application, I suggest to not modify the old modules, only connect new modules developed using SwiftUI.