If you use the second m.2 slot the bandwidth of the pci slot your gpu is connected to will halve from x16 to x8 so I don't think you want to do that.
You have two options I reckon,
1. Buy a large nvme ssd and install it to the first slot where existing 250gb ssd is and sell the 250gb ssd
2. Buy a large sata ssd and connect it to one of the spare sata ports in the 5th image leaving your existing ssd in place as a boot drive with windows/office etc on it.