I would not base myself anywhere for that long. Yes, there is a lot to see and do in Reykjavik, but not for that long and it would mean you are wasting precious time backtracking. Iceland is best seen with quick one nighters, which I know is not what many people realize. We have been many times, so am very familiar with this.
What I would suggest though is this. If you are arriving from the US after an overnight flight, make the day of arrival easy. Pick up your car and head into Reykjavik. Check in to your hotel and head out on foot and see the city. In the morning head up to Snaefellsness. Since you don't want to move much, two nights here would be good and would let you slow down a bit. There is a lot to see in this area. Then head down to Reykholt and do the Golden Circle. Honestly that only takes one day, even adding more than just the usual big 3 sights. If you want to stay 2 nights, I would opt for a place closer to Selfoss for the night, and then with the extra day you can see more the GC if you run short on time the day before. If you have 7 nights, I would then add a night in Vik and then the last night back closer to KEF, which you could even do Reykjavik and have a bit more time there or see the Reykjanes Peninsula. The reason for being back closer to KEF is that you don't want to risk missing your flight for some reason. In winter, road closures are common, but even in the summer things happen like last summer when a bridge on route 1 washed out with glacial flooding.