I think KATO intends that 216mm R track for trolley-type short wheelbase cars.

I don't have any curves this tight in operation, so I can't speak from experience. But here's my best guess ...

Your NW2 by itself will probably be OK, and coupled to relatively short cars (40 foot and less) with truck mounted couplers (like off-the-shelf MicroTrains cars) should work. I have found that mixing car lengths and especially mixing body-mount and truck-mounted couplers can make things challenging on tight curves, particularly when shoving.

If it were my layout plan, I'd try out a few pieces of track and a variety of rolling stock on a tabletop before cutting wood.

The extra joints in Unitrack vs. flex are more places for wheels that are skewed by being shoved around tight curves with off-center forces from coupler offsets to derail. But if you don't plan switching moves that require shoving, of course you'll have fewer issues.

I have made a test with Kato NW2 on very tight radii and surprisingly it had more problem then many other locos. I tested up to 8" and there the NW2 had some problem. You have to test 8,5" before deciding!

The best locos for tight radii that I had was Atlas. All Altlas I tested worked fine on a radii as tight as 6,5". Se my test video: