In the network tab in my console I'm getting a 404 after selecting a option from the first dropdown. That's because of the $resource = $request->newResource(); which leads to the resource() method in the InteractsWithResources trait:

return tap(Nova::resourceForKey($this->route('resource')), function ($resource) {
    abort_if(is_null($resource), 404);
    abort_if(! $resource::authorizedToViewAny($this), 403);
});

$this->route('resource') is returning false because there is no resource route parameter in the route:

Route::post('/options', 'OptionsController@index');

Laravel: 5.8.10
Laravel Nova: 2.0.1