In my shop we use XAML for UI, you may have some luck writing fluent methods for C# UI code but I find the declarative nature of XAML to be fairly intuitive and easiest to use.
That being said you may want to explore Reactive UI for ViewModels. It's fluent in design right out of the box.
https://reactiveui.net/docs/guidelines/platform/xamarin-forms
var canExecute = this.WhenAnyValue(
x => x.UserName, x => x.Password,
(userName, password) =>
!string.IsNullOrEmpty(userName) &&
!string.IsNullOrEmpty(password));
LoadCommand = ReactiveCommand.CreateFromTask(LogOnAsync, canExecute);
LoadCommand.ThrownExceptions.Subscribe(error => { });