OK - I’ve been testing with that.
When using Serial over USB there’s not need to “end” it.
If not using USB (eg using battery pads); just don’t do a Serial.begin();
The nullptr is correct, but doesn’t matter as DeepSleep replaces the pointer parameter with nullptr anyway. A wakeup isn’t called with DeepSleep, just a device reset.
The very dim charge led is “normal”.
What other code have you added?