Did run Roon for several years on my QNAP 671 with 16 GByte and i5, I just have appr. 20 TByte with appr. 440.000 tracks, also lifetimer; I always had troubles running the core on my QNAP (security issue with the port of Roon to QNAP, remember this port is done by a guy in his freetime and not officially supported by Roon), refer to Roon stopping randomly, to be honest it might be that those problems were due to the combination of Roon and Linn as these problems continued with my NUC10i7FNK with 32 GByte where I moved the core to, refer to Roon on ROCK crashes due to Linn devices with "CreateHandle: Assertion" error [Ticket In] - #72 by fschmeis; for the last 4 weeks I am SO HAPPY with this combination, didn’t had any issue any more, love running Roon on my NUC and having the library on my QNAP; Running the core on a NUC and having a huge library on QNAP I see only two potential downsides: 1. initial analysis of the library might take some time => connect NUC with 1 GBit Ethernet; 2. adding new albums to the library might need to initiate manually a (fast running) rescan (I don’t mind)
Who am I to recommend something different than Dany from Roon 
, but what I would do: test it on your existing NUC and if happy in principle: purchase a brand new (refer to https://help.roonlabs.com/portal/en/kb/articles/roon-optimized-core-kit#More_about_ROCKs_hardware_support) NUC12WSHi7 with 32 GByte and a very fast SSD, install ROCK (supports for some weeks now also UEFI startup), connect your QNAP and NUC via GBit Ethernet and by happy for the rest of your lifetime 