You can use NavigationPage.TitleView instead of Toolbar or Toolbar item.
All you have to do is just adding fontawesome libraries to each platform and than set font familiy property according of target platform.
<NavigationPage.TitleView>
<StackLayout Orientation="Horizontal">
<Button HorizontalOptions="EndAndExpand" Text="">
<Button.FontFamily>
<OnPlatform x:TypeArguments="x:String">
<On Platform="iOS" Value="Font Awesome 5 Free" />
<On Platform="Android" Value="fa-solid-900.ttf#fa-solid" />
</OnPlatform>
</Button.FontFamily>
</Button>
</StackLayout>
this is the one of the basic usage. For better usage you can create your own static resources and apply to button or what ever other items.
This is a code sample from one of my project:
<!-- Toolbar -->
<NavigationPage.TitleView>
<Grid HeightRequest="50" BackgroundColor="{StaticResource DarkBackgroundColor}" Margin="0" Padding="0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"></ColumnDefinition>
<ColumnDefinition Width="50"></ColumnDefinition>
<ColumnDefinition Width="50"></ColumnDefinition>
<ColumnDefinition Width="50"></ColumnDefinition>
</Grid.ColumnDefinitions>
<!-- Title -->
<Label Text="Inventory" Style="{StaticResource TitleViewTitleLabel}"/>
<Button Grid.Column="1" Text=""
Style="{StaticResource FontAwesomeButtonRegularTitleViewButton}"
Command="{Binding SyncDataCommand}"/>
<Button Grid.Column="2" Text=""
Style="{StaticResource FontAwesomeButtonRegularTitleViewButton}"
Command="{Binding ShowFunctionalityCommand}"/>
<Button Grid.Column="3" Text=""
Style="{StaticResource FontAwesomeButtonRegularTitleViewButton}"
Command="{Binding AddNewInventoryCommand}"/>
</Grid>
</NavigationPage.TitleView>