Hi @mynameiscris !
Nice plan I tit a small look see
For your day 1, I'd consider doing Krysuvík and all that before heading into the city to minimise driving. Also make sure to check the conditions at Skátalaug well, and never attempt to enter Leiðarendi witout a guide. As for groceries Go to Bónus. It's next to smáralind mall but cheaper
On day 2 I'd consider skipping hverargerði, as it does put quite a loop on your way., Maybe rather enjoy some time in the Borgarfjörður area?
Then, in Snæfellsnes, don't forget places like Hellnar and Arnarstapi. it's amazing there! You can also go horseback riding from Lýsuhóll
As for Hofsós, you're pretty well set though driving from there to Mývatn is a bit long. Also, If you're not getting someone to book for you you should look into Mývatn accommodation like, NOW (if you can, in Skagafjörður, go to Grettislaug baths, it's amazing)
In Mývatn, you should not miss places like Dettifoss falls, and take a part of the day to go to Ásbyrgi canyon. Also, if you like, the best whale watching spot in Iceland is Húsavík, which is nearby
There is one huge problem though: planning to drive Mývatn-Reykjavík in one day AND have activities AND do the blue lagoon. - I'd not recommend it. it's just too far and too much. The roads here are not smooth, and this takes like 6 hours if straight driving. You should consider rather going to the Blue Lagoon upon arrival and then driving atleast half way back the day before your penultimate day.
Hope this helps!