A nuclear weapons "test" would also be a violation of the Comprehensive Test Ban treaty, which Russia is a signatory to. I wonder what China's response to that would be.
An IRBM strike on Eastern Europe, most likely Poland, brings its own complications though. An IRBM risks being intercepted by AEGIS ashore, THAAD or Patriot. A cruise missile is far from guaranteed to make its way through NATO air defenses.
Why does a NATO response to Russian nukes need to be nuclear? An unrestrained NATO air campaign over Ukraine with the threat of conventional cruise missile strikes on strategic targets in Russia would be far more damaging.
Holding that line is going to be a big ask though. There's no obvious natural obstacle like the Oskil river to form a line on this time. If they try and hold Svatove with nothing on either flank, then the chances are the Ukrainians will just go straight round.