fix maybe?

configuration.nix

@@ -82,7 +82,42 @@
     noto-fonts
     noto-fonts-cjk-sans
     noto-fonts-color-emoji
-    iosevka
+    (iosevka.override {
+      set = "Custom";
+      privateBuildPlan = ''
+                        [buildPlans.IosevkaCustom]
+        family = "Iosevka Custom"
+        spacing = "fontconfig-mono"
+        serifs = "slab"
+        noCvSs = true
+        exportGlyphNames = false
+
+          [buildPlans.IosevkaCustom.variants]
+          inherits = "ss12"
+
+        [buildPlans.IosevkaCustom.weights.Regular]
+        shape = 400
+        menu = 400
+        css = 400
+
+        [buildPlans.IosevkaCustom.weights.Bold]
+        shape = 700
+        menu = 700
+        css = 700
+
+        [buildPlans.IosevkaCustom.slopes.Upright]
+        angle = 0
+        shape = "upright"
+        menu = "upright"
+        css = "normal"
+
+        [buildPlans.IosevkaCustom.slopes.Italic]
+        angle = 9.4
+        shape = "italic"
+        menu = "italic"
+        css = "italic"
+      '';
+    })
     nerd-fonts.symbols-only
   ];
   users.users."lucy" = {

stylix.nix

@@ -49,8 +49,7 @@
         name = config.stylix.fonts.monospace.name;
       };
       monospace = {
-        package = pkgs.iosevka;
+        name = "Iosevka Custom";
-        name = "Iosevka";
       };
 
       emoji = {