Try systemd-console mode "max"

extra-systemdboot.nix

@@ -2,6 +2,18 @@
 {
 ##### systemd boot use bigger console text #####
 
-boot.loader.systemd-boot.consoleMode = auto;
+boot.loader.systemd-boot.consoleMode = max;
+
+# The resolution of the console. The following values are valid:
+#     "0": Standard UEFI 80x25 mode
+#     "1": 80x50 mode, not supported by all devices
+#     "2": The first non-standard mode provided by the device firmware, if any
+#     "auto": Pick a suitable mode automatically using heuristics
+#     "max": Pick the highest-numbered available mode
+#     "keep": Keep the mode selected by firmware (the default) 
+# 
+# Type: one of "0", "1", "2", "auto", "max", "keep"
+# 
+# Default: "keep"
 
 }