Where should I base myself for a multi-resort road trip instead of picking one resort?

Pick a hub city with several resorts nearby and visit a different one each day, rather than committing to one hotel in a single area. It suits people who'd get bored lapping the same lifts all week more than it suits a family wanting to unpack once.

Iiyama is a clean example on the Nagano side: a base within reach of the Nozawa, Myoko and Madarao clusters without tying yourself to any single one. The real advantage isn't dodging one bad day, conditions swing day to day everywhere in Japan regardless, it's being able to plan each morning around the forecast: go high when it's warm and wet, drop low and sheltered when it's windy, rather than being stuck with whatever your one booked resort happens to be doing. Northern Honshu has an equivalent in Morioka, a proper city with a Shinkansen stop and enough compact but snowy resorts scattered around it.

The cost is time in the car, some mornings longer than others depending which resort you've picked, and it only really works with your own vehicle, ideally AWD, but definitely with winter tyres. Doing it without a car limits you mostly to the resorts that have a direct bus, which defeats the point. People do do it though, and it's completely understandable if you'd like to avoid the hassle of driving or the carbon emissions.

It also opens up resorts that don't have enough beds, are too expensive to stay in resort, or frequently sell out. For example, Kiroro works well from a base in Otaru, or you can visit the Hakuba Valley from Omachi. Kutchan is perfect for Niseko and Rusutsu.

Book the hub-city hotel first, and decide which resort you're hitting on which day once you're actually there watching the forecast. That's the whole advantage of this approach over locking in a single resort months ahead.