I too have a NUC5i3MYHE with 8GB RAM, with a 240GB M.2 SATA SSD and also asked myself that question, should I be getting a newer NUC for my Roon environment?
So I did some benchmarks - see here List Your NUC Capabilities Here
And the NUC5i3 did EVERYTHING, including serving 4 zones with DSP processed content (upsampling, downsampling, Room EQ etc.) and this was just as well as a later NUC and more powerful processors.
Much of the later evolution of the NUC range has been to support 4K displays and Gaming (not required for ROCK), having in-built WiFi and Bluetooth (not required for ROCK)
Much of the evolution of the Intel CPUs is to support higher clocking (NUCs only use the mobile versions of the CPU with the lower TPWs)
Intel has not really added much to the central core processing elements of the CPUs - the i5 and i7 are more powerful versions (btw just tighter tolerance versions, which can be driven harder at higher clock speeds, but consquently generate more heat, which is unrequired in a NUC).
Intel has been unable to drop the chip die size from 14nm to 10nm, so any processor from 2015 in terms of the core CPU processing pipes is exactly the same, despite the marketing hype.
So for an embedded appliance application, such as ROCK which runs headless, there is absolutely no requirement for later NUC generations or faster processors.
But please review the Benchmarks - follow the links to the device comparison table in that thread.
But for a library of 5k albums, save your money, and buy and enjoy more music.