Is zombie mode that powerful? is there another mode out there that takes care of the DAG sitatuon?
This is only possible if you already have a motherboard that has several PCI-e x16 connectors. However, buying a dedicated motherboard is not good because the price of a motherboard with 7 PCI-e x16 connectors is over 400 euros. Also the processor must support a lot of PCI-e lines, e.g. i7 or i9 with 40 PCI-e lines (e.g. socket 2066 etc.). You know that the processor for that socket is extremely expensive. However, even then you will not achieve full speed on all graphics cards but only for two and a half speeds for the third (2 x 16 + 1 x 8 = 40 PCI lines).
Yes this is correct, even if there was a motherboard which had 6x of the x16 connectors, it would not have enough lanes which is a limiting factor of the motherboard chipset and CPU.
This is why if you got 1 GPU and plug it into the x16 slot, it uses 16 lanes. However if you got 2 GPUs, then both of them run at 8 lanes each. The first GPU downgrades into 8x. Most people don't realise that just because its a different slot doesn't mean the lanes are indepedant. The lanes are shared between the slots.
I realised this when I would fill up an entire motherboard full of GPUs and one of them wouldn't work. Upon investigation it turns out that the lanes are shared and most of the x1 are disabled if others are filled. Hence why you should always read the instruction manual which has a diagram for all the available lanes.