for this itin, you probably do not need the Hokkaido Expressway pass
the long drive with Eway is from Nobo to Hakodate, but the non-toll road is just as good and scenic
u can get the ETC if offered, if not, does not matter much as the 30% off toll on weekends & PH.
In Otaru, there is a less touristy seafood market Nantaru Market. Not bustling like Sankaku but more wallet friendly.  Local aunties go morning shopping and salaryman lunch here.
After Kamui, u may want to check out Yoichi Nikka Whisky Factory, there is a cafe that serves good set lunches.
I hear now have to register online for free whisky sampling.
There is also a popular seafood restaurant/supermarket nearby within Yoichi.
Shibazakura - when in season, they r everywhere.  For your itin, best place to view is Mishima's Garden in Kutchan and hopefully get to see Yotei in background.
I have not been to Rusutsu.  I usually stay at Niseko Northern Annurpuri Onsen Resort .. damn worth it, for May shld be around SGD 120 per nite for 2 pax with breakfast.  The BF serves good quality stuff eg) on site fresh baked croissant with home made jam
In the nearby town of Makkari, there is a highly rated local pork chop restaurant .. ichifuji. There are no tourists here ( the shop has a sign to say they dont speak English ) but we tried 3 times, either sold out or closed 

 We did enquire inside before, and they do serve foreigners,just use G-translate.
Lake Toya - avoid Nonokaze, all the hype, costly but not worth.  Avoid Sun Palace - lousy room n food.
7 days, with Sapporo n this itin, a bit of touch n go I feel ...