I had the same issue with NVMe, but you can get 4x PCI-E cards to install them on. Wouldn't work as a boot drive but it's great for a games installation space. Just check installing another card doesn't drop your GFX from the 16x. My old ASUS Gene V had two 16x style slots and an extra 4x so I could manage to add one card without dropping the GFX to 8x.

My XBOX has the internal 1TB then 4TB of USB3 storage for 360/XBOX One games (or Series games I'm not currently playing). Main PC is 500GB SSD Raid 0 System drive, 1TB NVMe data drive and a 6TB HDD Storage Space. Then my NAS has 20TB (RAID 5) of space for everything else. (Including a full set of install files of any games I bought via GoG since you can keep their DRM free install files just in case GoG suddenly disappear)