Sounds a bit like the system on the twin I fly. The screens can only be bought on before start by using bus shed. I was warned against having them on before start due to potential spiking as the invertor comes on line. I can however put a flght plan in the system as the magic box for that works off the battery.
The 2-3 minutes after start is used for the remaining checks and then I set the screens up as I wish. I understand that the EC155 has a shortage of battery power for having all the toys on before flight, I'll find out for certain when I do the course in a couple of months. Enjoy your 2-3 minutes, they are very useful for reminding yourself that everything is done and ready.
VH