I assume that you have to use Chitose because of a direct international flight there. In case you fly domestically, I recommend to use Asahikawa Airport on one leg to save just a little bit of time.
You might already be aware of it, but one of the most famous fruit farms is Sobetsu Farm near Toyako:
arr Chitose airport late evening, overnight at or near airport but don't know which hotel yet ?
There is one inside the airport:
then driving to Sounkyo onsen, expect to arrive in the evening. How long roughly to drive using the exp'ways as much as possible and that day being a public holiday ?
It is a long distance! About 4-5 hours almost entirely by expressways, not including breaks. Considering that it is the last day of a long weekend, I am wondering whether there might be some traffic jams around Sapporo (I am not familiar with the traffic situation around Sapporo), which could cause some delays.
To reduce driving times a little bit, you could consider visiting Noboribetsu after Toyako and Furano before Sounkyo.
July 20 +21 : unplanned, was thinking of either staying longer in Sounkyo or visiting Asahidake or Teninkyo onsen, any suggestions ?
Teninkyo is not very nice. I think it depends on your interests. For example, if you like hiking, I strongly recommend to go hiking in the Daisetsuzan mountains with Sounkyo and Asahidake as possible bases.
leave in morning for Chitose, airport, what is the estimated driving time ?
Furano to Chitose Airport is about three hours via Takikawa and expressways. An alternative route via Yubari is somewhat slower, but cheaper.