This may or may not work. ROCK is targeted solely at the NUC platform so depending on drivers you might have an issue. Regardless, as mentioned many times above this will not be a supported configuration.
At this point we don’t know. @danny has made it pretty clear that Thunderbolt may or may not work and that all depends on proper driver support in the linux kernel.
Don’t count on it… no, let me rephrase that… extremely unlikely.
Overall, given the data rates involved I think you’re going WAAYYY overboard on this. DXD has a data rate of approximately 20megabits / sec. Most spinning drives can easily deliver 50 - 80 megabytes / second. USB3 has more than an order of magnitude more bandwidth than that. There won’t be bus contention. There’s way more bandwidth than traffic. Thunderbolt won’t get you or your friend anything aside from a lighter wallet.
Same goes for striping the SSDs. Get an NVMe M.2 card and you’ll have significantly more bandwidth than Roon will be able to use for its database.
Regardless, unless you’re planning to build on a NUC you aren’t doing your friend any favors by trying to load ROCK on unsupported hardware.