It will be a great trip @Philphed
you will have so much pow!
My suggestions: do not book all rooms before hand. Stay flexible. Do book a car, and yes, 4WD with wintertires is a must.
Follow the weathersites closely. Because of the terrain, especially in the Niseko area, the westside of a mountainarea may get dumped on a metre in a night while the southside got not even half that.
In the Niseko area there are some quiet spots to find, either near the passes that are closed in wintertime, or at the backside of resorts where you need skins to get in or out (or both) or the route. So bring your touring gear. The Yotei is not the only volcanoe you can climb btw. (They can all be climbed, provided the government will not have warranted a warning because of heightened activity.. this you can also find on the japanese weather websites)
Snowboarders can opt for snowshoes if they want, they are generally for hire in the japanese skiresorts and not expensive, but slooooooowwwwwww... a splitboard is better.
Do not skip Daisetzuan national park (south of Asahidake) but here again: bring your skins. There are no lifts there. that is the beauty of it. And; Rusutsu. But anywhere there are goodies to be found.
Do not expect gnarly alpine terrain. Do expect powder up your nose, though.
In my experience, all american and japanese airlines are impossible to bring your ABS or Snowpulse onboard. I lost three canisters that way. They just do not do it. Oz/Newzealand and European airlines are usually okay, but do bring the IATA forms and explanations etc etc.
Enjoy the onsen! Rotemburo's are even better (outside onsen)
I think it is worth the hassle to bring your own gear. Totally, absolutely worth it. SKINS!