Trip to Hokkaido in January

I am going to Japan for the third time in January this year. This time I intend to spend a week or just under in Hokkaido. I will fly in to Tokyo and spend a few nights there before heading north to Hokkaido. I know that the trip from Tokyo to Sapporo is about 10 hours which is a very long trip on a train especially when there won't be anywhere to charge my phone or laptop. I am willing to commit a few days to stopping somewhere in between to reduce journey time.

I have thought about staying in Sendai but can't decide whether it's really worth it, and if I do choose to stop in Sendai how many days I should stop there. Alternatively I could stop at Aomori but that place doesn't seem very remarkable either. Yet of course it's much closer to Hokkaido than Sendai is. I only have 19 days in Japan and after Hokkaido I need to go to the Kansai region. I know this is an even longer trip so I'll definitely need to stop somewhere in between. I feel like stopping in Tokyo would be a waste of time since I've spent so much time there previously, so does anyone have any suggestions as to where I could stay one night at least? I should state that I have a 14 day rail pass so flying would be a waste of money.

Another question I have is about the weather in January in Hokkaido. I asked Siri today and she told me that it's already below zero in Sapporo and it's just the start of December. I can imagine it will be absolutely freezing when I arrive in Sapporo, and it will be covered in snow. How thick can the snow get in Sapporo? I am from Auckland, New Zealand so I'm not really used to snow. Am I likely to need special boots to walk through the snow?

I am willing to commit a few days to stopping somewhere in between to reduce journey time.

What exact dates?

I highly recommend Hakodate:-
http://www.japan-guide.com/e/e5350.html

If you catch the early morning Shinkansen from Tokyo you arrive at Hakodate at 12:30pm or so, plenty of time to explore the city. The tourist office at Hakodate is also very helpful. I highly recommend staying for the morning markets before continuing on to Sapporo/wherever.

after Hokkaido I need to go to the Kansai region

I'd strongly consider flying. Jetstar Japan flies Sapporo to Kansai Airport for around 5,500 yen which may be a worthwhile spend to save you essentially two days.

There are heaps of other options in the Tohoku region if you wish to stay somewhere on the way back. Alternatively, going from Hakodate back to Osaka is possible in the one day and not too bad. We've personally done Hakodate to Okayama (past Osaka obviously) in the one day including lunch in Hachinohe and some quick shopping in Ameyoko (Tokyo)!

Enjoy your trip!

I can imagine it will be absolutely freezing when I arrive in Sapporo,

Yes.

and it will be covered in snow.

Yes.

How thick can the snow get in Sapporo?

Very thick although obviously roads and sidewalks are swept/cleaned.

I am from Auckland, New Zealand so I'm not really used to snow.

A Kiwi not used to snow?

Am I likely to need special boots to walk through the snow?

We didn't use any when we were there but you may wish to take some. You can also get clip on spikes/studs if you wish.

It's possible trip and I had done it before in February.
First trip is from Shin-Osaka to Hakodate and second trip is from Sapporo to Tokyo.