Am i too early to plan for my next year summer vacation?

It will be my 4th visit to Japan, but my 1st one to Hokkaido. I initially plan to do it this year, but too last minute, too difficult to get leave, and flight ticket is abit expensive.. thus need to keep an eye on the Airasia coming sales =)
so few question here to clear my doubt

-What is the best time to visit Hokaiddo during summer? main concern is weather - dont want too hot
i plan for mid-late July visit to avoid the golden week, is it ok? It should be lavender/ sunflower peak season right?

-Tentatively, we will travel in a group of 2 adults, 1 elder, and 2 kids (4years old and 2 years old), i assume best way for transportation is by renting a car? Do child car seat (for 4 years old) is mandatory? SUV/MPV will cost me much more than a sedan...

-will only spend around 7+ fulldays in hokaido (exclude the date of arrival/departure), i hope i can stay longer though.. the concern is $$ and the stamina of the kids/elder.

-tentatively -> Asahikawa/Biel/Furano - Sapporo/Otaru

-plan to stay a night in a ryokan with onsen, any recommendation ya? so many hotspring spot scatter around Hokkaido..

-I found the accomodation in Biel/Furano is quite limited / not family friendly.. Day trip from asahikawa doable?

U should be able to renew your passport in Malaysia consulate/ embassy in the state.. check with your Malaysian senior there? I did that in other country

Anyhow if the passport is expiring in less than 90days, I wonder if the us emigration let you pass through?

Plan to visit Hokkaido during spring 2018, is it possible to see sakura and shibazakura in 1 go? tentatively 2nd week of may(to avoid golden week), travel time 10days.