Hi! I like the idea of only staying in 2 different places, and I also understand the difficulty of picking what those 2 places are!
First, I’d skip the Westman Islands. The close port (Landeyjahöfn) may not open until mid April, and closures are more common in spring and fall. That would leave you with an almost 3 hour ferry ride each way, which isn’t practical for a day trip.
There’s not right answer. But maybe use day 4 to get past Vik? Since you’re not backtracking, there would be less driving. Cover Seljalandsfoss waterfall, Skogafoss waterfall, Reynisfjara and Dyrhólaey as you drive east, and maybe make Kirkjubæjarklaustur your 2 night base? That would make the roundtrip to Jökulsárlón more reasonable.
One other thought is to make sure you want to head all the way out to Jökulsárlón. Skaftafell is great, but it’s a long uphill hike for a 3 year old. If you decide to make Kirkjubæjarklaustur your terminal point, then staying in Vik can make sense and lessen your total driving.
If you have any time on your last day, consider the Blue Lagoon or other sites in the Reykjanes Geopark.
Good luck!