Do you have the Cicirone book? Walking and Trekking in Iceland by Paddy Dillon.
He lays out lots and lots of options.
I’m doing the classic Laugavegur & Fimmvorduhals this summer, and his route description seems useful. I’m taking it leisurely, conservative-mileage days and staying in the huts, so that’s a 6-day hike (which I prolly could do in 3). Though it might be 4 if the weather is crummy when I wake up in Thorsmork– there are some sketch bits on the extension to Skogar.
But, from the book you can see this super-popular, well-known route is just one of many, many options to choose from. Like if you don’t want to hike with 40 of your new best friends.
So I may have to go back for a second trip.