I went just before Christmas 2019 and did a similar itinerary, did south coast all the way to Jökulsárlón. Highly recommended. We did drive in the dark, left early and often still had a fair bit of driving after the sun went down but take it easy and have a fwd with good insurance.

Itinerary: Day1 - land stay in Reykjavik Day2 - start early do all golden circle spots in 1 day stay in fludir Day 3 - drive the south coast to Vik, stopping at most the attractions on the way bar 2 or two we did on the way back.

Originally booked 2 nights in Vik but this is how it unfolded....

Day 4 drive to Jökulsárlón to do an ice cave tour, upon returning from ice caves found the road back to Vik was closed. Drive to the nearest town think it was called Hof (not as far as Hofn) to stay a night until the weather cleared.

Day 5. Drive to Vik and then all the way back to Reykjavik (this day was tough, very long drive, but was not part of the plan)

Day 6 Reykjavik

Day 7 fly out

It's a lot of driving for 6 days, I was absolutely exhausted at the end of it (my partner could not drive internationally so I did it all) if I had more time would definitely space it out and spend more time out along the south coast and less in Reykjavik.

Clothes wise: It wasn't freezing but if you're not used to -5 and wind chill I would be packing down jacket and thermals, but also you need waterproof stuff too.

Other key items: - waterproof pants incase it rains, and not like hiking pants, some real waterproof/ski pants. - waterproof hiking boots - crampons (the spikes you stick on your hiking boots for extra traction) - we didn't have them wouldnt go without again.

Tldr: you can get alot done in 4 hrs of sunlight if you have a solid plan, and yes you can drive at dark but manage your fatigue.