Mario My understanding is that GDExtension support is included in the default export templates, so nothing has to be rebuilt from 0. This should naturally include all platforms that Godot currently supports. C# interop with GDExtension - seems like it's not possible right now to do directly, have to call GDScript from C#.
C# integration itself is up in the air and will probably introduce breaking changes down the road when they finally port it to GDExtension bindings. See C# roadmap discussion: godotengine/godot-proposals7895 But this is likely on the timeline of years, not months. And I can't comment on W4 console plans.
The docs are sparse indeed. And it seems like there'll be a ton of changes in the short to midterm.
EDIT: GDExtension should support all of these platforms.
From .gdextension in godot-cpp:
[libraries]
macos.debug = "res://bin/libgdexample.macos.template_debug.framework"
macos.release = "res://bin/libgdexample.macos.template_release.framework"
windows.debug.x86_32 = "res://bin/libgdexample.windows.template_debug.x86_32.dll"
windows.release.x86_32 = "res://bin/libgdexample.windows.template_release.x86_32.dll"
windows.debug.x86_64 = "res://bin/libgdexample.windows.template_debug.x86_64.dll"
windows.release.x86_64 = "res://bin/libgdexample.windows.template_release.x86_64.dll"
linux.debug.x86_64 = "res://bin/libgdexample.linux.template_debug.x86_64.so"
linux.release.x86_64 = "res://bin/libgdexample.linux.template_release.x86_64.so"
linux.debug.arm64 = "res://bin/libgdexample.linux.template_debug.arm64.so"
linux.release.arm64 = "res://bin/libgdexample.linux.template_release.arm64.so"
linux.debug.rv64 = "res://bin/libgdexample.linux.template_debug.rv64.so"
linux.release.rv64 = "res://bin/libgdexample.linux.template_release.rv64.so"
android.debug.x86_64 = "res://bin/libgdexample.android.template_debug.x86_64.so"
android.release.x86_64 = "res://bin/libgdexample.android.template_release.x86_64.so"
android.debug.arm64 = "res://bin/libgdexample.android.template_debug.arm64.so"
android.release.arm64 = "res://bin/libgdexample.android.template_release.arm64.so"
web.debug.wasm32 = "res://bin/libgdexample.javascript.template_debug.wasm32.wasm"
web.release.wasm32 = "res://bin/libgdexample.javascript.template_release.wasm32.wasm"