Have a look at my recent trip report on Nagano and Niigata. Some additional info.
When to go: I've always had good snow end of January in last five years. In 2016, I was there mid-Jan and encountered heavy rain crust on the southern aspect (bad creme brulee with too thick of a sugar later). Other aspects were great.
AVOID Chinese New Year, the entire week. Next year, that's Feb 5. So, perhaps last week of January. That keeps one in the heart of the mid-Jan to mid-Feb window, and out of the Chinese holiday
Where to Stay: I've always stayed at Shirouma-so, an up-to-date ryokan with its own onsen, in Happo Village. A short walk to Happo-one gondy, and, if you know the short cut, to Wadano Base. But, real close to the Happo bus terminal, making it easy to schlep both gear and bags. But, there are lots of great places to stay, including at Wadano, where Evergreen Backcountry is, also known as Evergreen Outdoor Center, a guiding operation that offers both off-piste (Sidecountry) and backcountry tours.
At Myoko, I stayed at a foreign owned and operated lodge named Morino Lodge. Careful, there is more than one, one being at Hakuba. When I return, I will stay at a ryokan, but this one has no ensuite baths, only guest-onsens, and no western beds.
HAKUBA SKIING
Resort and Sidecountry: Great skiing can be had on-resort. Happo-one has so much terrain relative to numbers of skiers, on a weekday, that one can have a great powder day in-resort for much of the day, and then the next day. I love Cortina for an in-resort powder day. Great tree skiing in-resort and sidecountry.
When things get tougher, one can find great snow in the lift accessed (not "served") Sidecountry. Resorts have become far more liberal about resort skiers going into the non-patrolled sidecountry. I think it is Tsugaike that mandates one attend its intro and safety session before one can enter. Regardless of where, one had better go with ski partners who know how to use their beacon shovel and probe. One had better go with a guide, or a friend who knows the territory, or you could end up in terrain traps, or further into the backcountry where a long boot pack may be necessary, shorter if one knows the way. What I like is that, at a place like Cortina, the Sidecountry can be the opposite aspect from the resort, in the trees. Great place to find lighter and less tracked powder.
Backcountry: Amazing BC skiing. The valley has huge amounts of terrain. However, some of the more easily accessed get tracked up in a few days, as many foreigners can access, online, routes others have posted. But, tracked up is relative. The powder is still amazing and, relative to a resort, not tracked much. The issue may be more about the tracked up hike up. Two of my best BC experiences were those for which we, with a guide, placed a vehicle at end of day destination, amidst farmland. Most foreigners are not going to have that facility, and would not know where to park.
MYOKO SKIING
See my trip report, as that is my only experience at Myoko, having only skied at Hakuba until this year, and only at Akakura Kanko. There, Sidecountry appears to me to be on the same aspect as the resort. The other side, one has a long climb.
Skiing in Japan is a great experience and very worthwhile doing, over and over again.