Hi @Alan_Meldrum,
Apologies for the delay here!
I believe your current Nucleus with additional RAM should be completely fine, although the Titan would also certainly do the trick as well.
You can add an internal SSD of up to 8TB capacity, which would strictly hold your local library. There is a separate m.2 SSD that hosts your Roon Server database.
Many users have used Samsung SSD 860 EVO 2.5 Inch SATA III Internal SSD, here’s a link to amazon for reference: