If you are renting a car from Blue, they are literally right across the parking lot, so 5 minutes tops of a walk. There are other car rentals, and some have shuttles, so either way it should be very quick to pick up the car. Returning at Blue, it has always taken us about 5 minutes to return it.
Coming home from Iceland, you want a full 3 hours at KEF. This is the airport run down. As you enter, Icelandair has kiosks right inside so you can check yourself in with a quick scan of your passport, and print luggage tags for any bags you will check. Put the baggage tags on and then head towards the check in desks, but go to the left of those and that is where your self tagged luggage will go. They have a scanner so you scan your tag, put your bag on the conveyor and off it goes. Then you go upstairs to security. You will need to scan your boarding pass before you get there. Security is pretty quick. Then it dumps you into the duty free store and then out to where the restaurants/shops are. Many people hang out here waiting for the gate announcement. Don't do this! All flights to the US go out of the D gates. The gates are not announced until about 45 minutes before. So grab a bit or shop, but then head over to Passport Control when you see the line is short. This is where people get caught off guard as the lines can be very long. So do this when there is a lull. After you go through there, you scan your boarding pass yet again and then go downstairs to the D gates. There are more shops and restaurants down there. Seating is very limited, so you might just hang out in a restaurant. The gate announcements usually come to our phones before they are on the boards in the airport. Once the announcement is made, head over to the gate. Boarding is a bit of chaos. There will be 2 lines, Saga and regular. Get in line early. Agents will come around looking at bags to make sure they are proper carryon size and if not, they will make you check it. Most of the time, the planes are not right at the terminal, so after you get your passport/boarding pass checked, you will go into a small entry area and load up into a bus. When you think there is no more room, people will still squeeze in. The bus will take you out to the tarmac to the plane and then you board.
For a 4:45 flight, I would return the car about 1:30.
Our 8th trip is coming up soon, so I am very familiar with all of this, lol.
I would suggest not coming from Vik on your last day. Weather in September can be anything, and there is a pass you go over as you head south from Vik. So if winter weather hits, you could easily get stuck there and miss your flight. It would be wise to stay closer to KEF that last night, even like Selfoss etc.
If you need any help with planning, just ask:)
With 8 days, you can do more than what you have planned. You can easily do the south coast up to Jokulsarlon, the Golden Circle and Snaefellsness. On my last trip, this is the amount of time we had and I was with a friend who had never been. It was not too rushed, and she got to see so much.
Lastly, you will notice a HUGE difference in the number of visitors from 2018! Our first trip was in 2015 and then we have just kept returning a few times a year since covid. Even from 2021/2022, the amount of visitors is astonishing.