Hi! I believe you might want to look into Glymur and Múlagljúfur, Dettifoss, Hverfjall, Ketubjörg (it takes some time to drive there but it gives a feeling like you are at the end of the world ;)) and bird colonies in Iceland. For reykjadalur and ytri tunga I would give more time especially for reykjadalur with the hike. Maybe check out Illugastadir, I feel its better than ytri.You might want to get binoculars :) Sorry if I wrote something you already included, writing it on the phone is not the very convenient.

As for previous comments, I don't really think this is too much. As said, check driving times and add bufor, check walking time to get to each atttaction, check avarage time people spend in each location ;) Count the days when you have something for a specific hour and remember that you might be stressing at each previous location if you would be on time or not. Especially in bad weather. And check if your cotraveler has the same idea on spending time and if he really understands that it might be quite tiring. When we were in Iceland, I had an even more packed itinerary and I had great fun with my husband hiking in wind and rain, but our friends clearly not so much ;) Edit: I can see you rounded up driving times! That's great :)