On the time allowed for the trip...
Yes, you proposed pace is doable, even reasonable. I have found that on trips where I averaged more miles than that per day I still managed to have lots of conversations with the locals, enjoy the scenery, take a ton of pictures, and generally enjoy the trip.
It is also possible to take twice as long and still not do that stuff. Some folks manage to maximize their spare time and others spend their spare time sleeping in or sitting around in camp. How much you manage to "smell the roses" is not strictly governed by how many miles you ride.
On the other hand do take into account that the spacing of services on the route will mean that at times you may need to take days that are either a lot longer or a lot shorter than you want.
FWIW, I think that different routes have different ideal paces. For me the TA is one where I'd take a little longer and the ST is one where I'd roll with longer daily mileage.
Oh, and one other thing... When it comes to budgeting time (and money) for the trip it is always nice to allow extra. A tight schedule or a tight budget can take some of the joy out of the trip. If at all possible allow extra time then finish when you finish whether that is way ahead of or way behind schedule. For me 10 weeks is about right for the TA, but I'd budget at least 11 and finish somewhere between 9 and 11 weeks Since you are from Iceland that may not be possible, but maybe you can have a little time allowed for some optional sight seeing at the end of the trip.