There should be no problem about using a second NUC with ROCK installed as an endpoint, particularly if you want to use the HDMI connection to feed a surround sound system.
You’re just not using the Core component of Roon, only the Output component.
Don’t forget that Roon installed on a PC or a Mac will also have the Core component present, but it’s not being used when the Core is running on another device (e.g. a Nucleus or another RCK/NUC)
Yes, you could use Linux and RoonBridge, but then you would lose the “appliance” aspect of ROCK.
Danny confirmed this use of a second ROCK/NUC was OK some time ago.