To be honest, none of the hiking routes in Iceland are crowded. Even the most popular hiking route, Laugavegurinn, that goes from Landmannalaugar to Þórsmörk (it’s 55km and people normally do it in 3-4 days) or the second most popular hike, Fimmvörðuháls from Þórsmörk to Skógar (22km and takes about 10 hours) are not exactly ‘crowded’. Yes, a lot of people go there, but on each day you might meet around 20 people max. Mostly when you reach the campsite at the end of each day. During the hike itself you only pass very few people.
So pretty much anywhere you go you’ll find a place that isn’t very crowded
Here’s a handy article to give you some ideas of where to go: Hiking in Iceland
Have fun - and remember to dress warmly in layers even though it’s ‘summer’