Sorry for the late reply. You can replace the optical drive with a 2.5" HDD or add a sata PCI-E card since your not using the GPU slot in this case. My personal setup uses an NVME for the OS (DietPi), and two 6TB external HDDs on an Optiplex 5060 SFF.
I only boot windows for Fortnite and The Crew 2 because of BS DRM. Everything else runs great.