Cycling Sapporo to Tokyo - weather conditions

Me and a friend want to cycle across Japan next year, from Sapporo to Tokyo.

This year we went from Hiroshima to Tokyo in late April to the middle of May and the weather was great - mainly dry and sunny, some rainy days and no humidity.

If we do this trip from Sapporo to Tokyo, which is more in the north of Japan, what time of year is best in terms of weather?

I am concerned that in March / April, it could still be rather cold up in Hokkaido.

So May is probably the best time? But then I've heard May can be a bit warmer but more chance of rain in the north.

I was also thinking we could do October instead and witness Japan in the Autumn, but I'm not sure what the weather would be like then.

Any help much appreciated.

I have ridden in Kansai & Kanto in mid March and the mornings were bitter although after about 10 o'clock ish it was lovely. Obviously the north would be colder. I am planning a cycling holiday to Tohoku myself & think Early-mid October sounds about right. A quick search on Sendai weather for last year shows highs of about 20 Centigrade & nightime lows of about 9. Bear in mind that mid October on would coincide with the peak autumn tourism season so early booking of accommodation would be wise.