I have developed a few cross platform apps, using different methods.
If you have over $1k burning a hole in your pocket, then I doubt that you can beat Embarcadero Delphi.
Delphi™ is the fastest way to write, compile, package and deploy
cross-platform native applications.
Delphi is the choice for developers wanting the power, readability and
flexibility of the Modern Object Pascal language, coupled with native
compilers and component libraries for fast single source code
development on Windows, macOS, iOS, Android and Linux.
Code once, run on multiple operating systems. It has an excellent drag & drop UI designer, plus tons of other features, as you would expect for the price.
Apache Cordova is free and excellent, especially if you want access to the camera, address book, etc.
For those that don't require access to the camera's hardware or any software resources (such as contacts), I just go with a browser based solution, as that is most likely to be truly cross-platform.
Personally, I code in Angular Js and use Ajax to communicate over a RESTful API to the server, which I code in PHP, but feel free to find your won solutions (e.q. jQuery & Node.Js)