The fact that all the lights dim before the headlamps, or flashers, come fully on is not a problem showing up – it is built in to the lights because the tungsten filaments have a cold resistance which is much lower than when hot. So, to start with, the cold headlamp bulb or flashers draw too much current (for a second or two) and there is not enough current to feed everything else at the normal value. There is no 'spare' power available in the generator or the battery. Changing to LED bulbs (except for the headlamp) would mask the problem in two ways – the LEDs would need less current, and they are less fussy about the exact voltage.
Cheers, Tim
Edited By Tim Stevens on 02/04/2021 20:20:35
Edited By Tim Stevens on 02/04/2021 20:22:26