We stayed an hour or so from Reykjavik, and I wasn’t involved in finding routes, so I don’t know that much. I also don’t know anything of the length that you want (did it in a big group so everything was slow). The person who found the routes just did a lot of internet digging I think
This was OK: https://www.strava.com/activities/152605284
Seem to remember the descent being pretty fun. Had a hot stream: https://www.strava.com/activities/152605349
This was ace. Quite a bit of walking with a bike on my back (also we went the wrong way off the peak. Had to ride/slide down a ice covered slope)
https://www.strava.com/activities/152605409
The lower descent here was pretty good, lots of flow. It could be really crowded with walkers though due to the popular waterfall at the bottom. No need to do the top bit (i.e. the non loop). The views weren’t as good as some of the other rides imho.
https://www.strava.com/activities/153354969
I really don’t think we scratched the surface though. So much good stuff out there.