This is enough for mining.
If a coin has a dag file equal to 6GB, then you will need the same solution as was done for 4GB cards for mining Ethereum with a Dag file of more than 4GB.
This is a tuned miner and maerit board with PCIE 16x slot.
As a rule, a regular motherboard for $ 100 has 1 GPU - x16 mode, 2 GPUs - x8 mode + x8 mode.
You need to look for a motherboard like this
ASUS X99-Deluxe
(x8 x8 x8 x8 x8)
Why do you need a zombie mode? It is no longer relevant for 4 GB of video cards for Ethereum mining, the hashrate is minimal.