Hey it kinda works...

src/play.rs

@@ -8,8 +8,8 @@ impl Plugin for PlayPlugin {
     fn build(&self, app: &mut App) {
         app.add_event::<ServeCards>()
             .add_observer(serve_cards)
-            .add_observer(fill_cards)
-            .add_systems(OnEnter(GameState::Main), deal_table)
+            .add_systems(OnEnter(GameState::Main), deal_cards)
+            .add_systems(Update, deal_cards.run_if(set_added))
             .add_systems(Update, spin_cards);
     }
 }
@@ -68,6 +68,7 @@ pub(crate) fn reset_rotation(
 pub(crate) fn check_set(
     _trigger: Trigger<Pointer<Click>>,
     mut query: Query<(Entity, &Card, &mut Visibility, &mut Transform), With<Selected>>,
+    mut sets: Query<&SetNumber>,
     mut commands: Commands,
 ) {
     let mut cards = query.iter();
@@ -83,10 +84,11 @@ pub(crate) fn check_set(
                 .for_each(|(entity, _, mut visibility, mut transform)| {
                     *visibility = Visibility::Hidden;
                     *transform = Transform::default();
-                    let set_number = 1;
+                    let set_number = sets.iter().len() as u8 / 3 + 1;
                     commands
                         .entity(entity)
                         .remove::<PlayLocation>()
+                        .remove::<Selected>()
                         .insert(SetNumber(set_number));
                 });
         }
@@ -124,12 +126,16 @@ struct ServeCards;
 /// When requested, fill in empty spots on the board with cards
 fn serve_cards(
     _trigger: Trigger<ServeCards>,
-    cards: Query<(Entity, &DeckOrder)>,
+    in_deck: Query<(Entity, &Card, &DeckOrder)>,
     spots: Query<&PlayLocation>,
     mut commands: Commands,
 ) {
-    info!("Serving cards");
-    let mut target = (cards.iter().len() - 1) as u8;
+    info!(
+        "Serving cards from deck ({} cards left)",
+        in_deck.iter().len()
+    );
+    let mut n = in_deck.iter().len().saturating_sub(1);
+
     // Iterate over every x, y play location
     for this_x in 0..=3 {
         for this_y in 0..=3 {
@@ -139,8 +145,13 @@ fn serve_cards(
                 .find(|PlayLocation { x, y }| *x == (this_x as u8) && *y == (this_y as u8))
                 .is_none()
             {
-                if let Some((e, _)) = cards.iter().find(|(_, deck_order)| deck_order.0 == target) {
-                    // Set the selected card to this play location and remove from the deck
+                // If we have more than 9 cards on the board
+                let candidate = in_deck
+                    .iter()
+                    .find(|(_entity, _deck_card, order)| order.0 == n as u8);
+                n = n.saturating_sub(1);
+                // If that search was fruitful, pull the card from the deck and play it
+                if let Some((e, _, _)) = candidate {
                     commands
                         .entity(e)
                         .remove::<DeckOrder>()
@@ -148,7 +159,6 @@ fn serve_cards(
                             x: this_x,
                             y: this_y,
                         });
-                    target = target - 1;
                 }
             }
         }
@@ -156,15 +166,12 @@ fn serve_cards(
 }
 
 /// Trigger dealing cards when the game starts
-fn deal_table(mut commands: Commands) {
-    info!("Dealing table");
+fn deal_cards(mut commands: Commands) {
     commands.trigger(ServeCards);
 }
 
-/// Trigger serving cards when a new set is created
-fn fill_cards(_trigger: Trigger<OnAdd, SetNumber>, mut commands: Commands) {
-    info!("Filling cards");
-    commands.trigger(ServeCards);
+fn set_added(query: Query<Entity, Added<SetNumber>>) -> bool {
+    !query.is_empty()
 }
 
 /// When a card is added to the board, place it on the screen
@@ -213,7 +220,7 @@ pub(crate) fn check_for_sets(
                         _ => panic!("WTF?"),
                     }
                 }
-                _ => panic!("Impossible!"),
+                _ => false,
             }
         })
         // return the first valid set