I think you are definitely pushing it trying to use an i7 NUC with the size of your library. Even Brian Luczkiewicz (Roon’s CTO) seems to imply this in his post on hardware requirements for a good user experience:
If you’ve got a 4k monitor and 20,000 albums, look for a big scary Core i7 with plenty of ram, an SSD, and discrete graphics.
The advice from Roon Labs for very large libraries seems to be to go for a workstation PC build running Windows…
Edit: If you want to keep to the NUC form factor, then perhaps a model from the new (slightly larger, and certainly more powerful) Intel NUC 9 Pro range would be suitable?