Before you spend any money on the later Generations - see List Your NUC Capabilities Here for actual benchmark comparison of an NUC5i3 vs NUC8i7 involving upconverting, downsampling, Room EQ in mutli-zone playback - the NUC5i3 was able to match everything a NUC8i7 did as a Roon Server.
Remember that the NUCs used a Mobile Intel CPU, and other than Cannon Lake and Ice Lake architectures, all have been 14nm chip die and the same number of processing pipes since 2015, see https://en.wikipedia.org/wiki/Intel_Core and https://en.wikipedia.org/wiki/List_of_Intel_CPU_microarchitectures.
Also the higher clock speeds in the later Core Generations are irrelevant to a NUC, where the TDP is 15W or at most 24W, so can’t deal with the heat generated as a by-product of the higher clock speeds.
The focus in the later NUC generations has been for on-motherboard Wifi and Bluetooth, and improved GPU for 4k displays etc., which are irrelevant to a ROCK server.
Has Moore’s Law ran out for NUCs when used for an embedded OS (Lunix derived) single function processing device, just using ethernet connections, and core CPU processing?
Simon.