Lot of options mentioned here already, but I’d still like to mention one more: Godot game engine. It has quite comprehensive set of UI-components (for example Godot editor itself is made using it’s own UI-components). And being a game engine, it also has good 3D-support. Godot’s native scripting language is python-like GDScript, but you can also use C# with a .NET-version that supports C# in addition to GDScript.
Multi-platform support in my experience comes pretty much free with Godot. For example the external simulator-PoC works in at least Windows, Linux and Android (not counting steering wheel / force feedback. Haven’t tested if UDP works in Android either, but I think it should). It should also support C# on Android, but I haven’t tested it.
From the perspective of an occasional modder/contributor Godot would also be beneficial in that the editor doesn’t even need installing, just downloading a less than 100 Mb editor-zip, unzipping and running it is enough to get one going. Exporting the project (so that it runs without the editor) needs some additional downloads, though. And for an occasional coder GDScript could be easier than C#. For “higher level” operations it’s performance should also be good enough, although heavier operations probably should be done with C#.
Considering the precision needed for location, Godot editor may need to be compiled with double-precision floats (the internal classes can be compiled to use either 32 or 64-bit floats). In C# you can of course just use doubles, but GDScript, the engine itself and those classes will use the precision set when compiling. Currently there’s no ready-compiled editor executables for double-precision to download.
That said, there are still annoying issues with touchscreens that currently makes Godot unusable with at least some tablets’ touchscreens (like Panasonic FZ-G1).
Not sure about safety related issues, but don’t remember when Godot last crashed when running a project or so either. Editor has had some issues, although rarely.
The text above might be biased as for some things I have lately been using Godot as my “hammer”, so every problem looks like a nail. 
Sorry if it feels far-fetched.