Yes, it is doable. I just came back from my 7 days self drive Hokkaido trip Here's my brief itinerary

26 June - Night flight
27-28 June - Sapporo (2 nights)
29 June - Noboribetsu Marine Park --> Hell valley --> Lake Toya -->Hokadate (1 night) --> Hakodateyama Yakei (night view)
30 June - Hakodate Asaichi (Morning market) -->Kanemori Akarenkasouko (shopping)--> Otaru (1 night) --> Otaru Canal
1 July - Otaru's Rokkatei, Kitakaro and Letao (eat and shopping) --> Otaru Orgel House (Music box) --> Furano (1 night) --> Lavender/Flower/Melon farm
2 July - Unkai Terrace --> Blue pond -->Lake Kamui (1 night)
3 July - Back home

Note: i'm not recommend anyone to follow this super rush trip of mine, it is just too tiring especially you are the only one who is driving.

You can ask me anything about self driving in Hokkaido while my memory still fresh

Hey there, hope this help ( just came back from Hokkaido self drive)

1. for your D1 which route you plan to use? If same as mine, you will be reaching Asahikawa first, so you will need to drive to Furanu/Biei and back to your hotel (1 hour+ one way) Or the other route, you will be driving a lot more hours. How about staying in Furanu instead? Suggest you stay at least 2 night in Furano (if you are into Flowers, melons, lavender, country side and mountain)
2. Noboribetsu and Lake Toya are nothing special, worth a detour but not a full day, you may visit them while on your way to Hakodate.
3. Spend at least 2 nights at Hakodate, you won't regret it.
4. Skip Niseko
5 Day trip to Otaru but no need overnight. Stay at Sapporo instead. Sapporo to Otaru is just around 45 minutes. Spend 2 nights in Sapporo
6 Every souvenir, chocolate, or Hokkaido limited stuff are available at the Airport at around the same price. So, don't worry if you forgot to buy anything.