If you want to simplify development of a cross-platform app, your work should start by architecting the software in a way that the core business logic is agnostic to the user interfaces. Then your web, mobile, and desktop GUIs, CLI, TUI, API, and any other way a user interacts with your program are simply thin abstractions that call your core components.
The complexity of each UI will inevitably vary, and it might not be possible to offer a consistent experience across platforms or to leverage unique features of each platform without making your core a tangled mess, so this is certainly not "easy" to do in practice, but this approach should avoid the bulk of the work of targetting individual platforms than if you had done it any other way. It should also avoid depending on a 3rd-party framework that may or may not exist a few years down the line.