Japanese sleeper trains have been seriously cut back, unfortunately. There's the Sunrise Izumo and the Sunrise Seto which run from Tokyo, but very, very, stupidly, there is no sleeper train from Tokyo to Sapporo. The Cassiopeia was a top tier sleeper train that was cancelled before the Shinkansen even reached Sapporo. I believe they still run a few services each year as cruise trains, but those look to be exorbitantly expensive. Also lacking from the sleeper train roster is the lovely Hokutosei which was a more modest train that still had some very nice features.

I’m in Sapporo right now and it’s unfortunately nearly burger levels of car brain. The buses and trains run once or twice an hour with very limited routes, I couldn’t even go to the big national park just south of here via public transit. My gf was quite shocked at how poor the public transit here compared to Tokyo so just be aware that if you come to Hokkaido you really need a car