Every week counts (I've been for a month or so every year of the last 9 or 10, at varying times, all to Hokkaido).

The snow base required in Hokkaido is quite deep. Partly due to the sasa grass (bamboo grass leaves), partly the low altitude (so tress etc need to be covered, less so in alpine regions) and partly due to the light powder which packs really small.

It takes many metres of snow in Niseko and Furano to create a ski-able base. High season yes, but many in Niseko around that time are now Chinese too, and lots of the people around are newbies who stay low or are in lessons. Every time I've been at Christmas and New Year the queues in the early AM are ridiculous, but as soon as you rise up past them it's fine for the rest of the day.

Snow quality I agree will not change. But the base will. Then it also comes down to luck- it might pelt down the week prior and be soft at least, but just compact rapidly. Or it might come down when you are there. Always go as late as possible until early/mid Jan is my advice to people.