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Yes, I pretty much always do self drive tours. I also do itinerary myself and rarely use agancy for any help. Planning is another fun part of my trip :)
Summer is awesome in Iceland, due to the daylight as you already wrote. But winter has other charms. I somehow like winter edition of Iceland more. It just suits more to the landscape and its name I guess :) Or it might be due to Northern Lights :)
But it is definitely more pleasant to travel there during summer months.

Well I do hope I could see both summer and winter as you did eventually. Northern Lights aren't something to miss out on.