25k albums w/ Nucleus+ is fine. I believe @mike runs around that number with a less powerful machine.
In the worst case, you would need an extra stick of RAM, which can easily be upgraded later, but I’m not sure it’ll be needed for 25k albums.
There are only 2 exposed USB ports on the Nucleus+.
25k albums in CD quality FLAC should be about 10TB. You can get a single 12TB drive from Amazon for about $375. You could use an enclosure like the one below to put a pair of those drives into a single case for fault tolerance purposes (RAID Mirror) and attach it via USB to the Nucleus+ with a single USB cable.
…or you could get a NAS. I personally would prefer the USB drive solution as it works better with Roon.
While possible, USB hubs are known to be bottlenecks for performance, failure points, and overall introducers of unreliability into any system.