Given the other advantages of Nucleus+ (aesthetics, fanless design, and most importantly, it runs Roon OS), I would always argue that Nucleus+ is the best choice.
However, for 337k tracks, you might possibly run out of RAM. You can add a stick of RAM into the device after the fact if you want. I can walk you through that. We’ve purposely left a blank slot for exactly this purpose.
USB drive is easiest w/ a single 3.5" 10TB disk. But I would put the noisy spinning drive in the NAS far away from your listening room, and keep the Nucleus+ and your UltraDAC where the action is. Both are nice looking boxes, unlike the NAS.
You can always try both, it won’t hurt you to try, nor will it cost you anything but a few minutes. I’m unsure if the Sooloos network protocol can stream DSD to the UltraDAC – but that may not be important to you. If it isn’t, then use the network. If it is, you might need to use USB. @john, our hardware ecosystem expert, should be able to chime in with information about that functionality.