It feels like the Endurance series fails to attract players most weeks, and I think it’s due to the classes more than the time.
Forza GT and LMP (when it’s offered) are much more popular than the other playlists. And the LMP1, LMP2, GT2/3 series does have generally more full lobbies.
At some point, maybe have two endurance series, with the LMP1/2 GT2/3 race permanent, along with the other classes in rotation?
Maybe that permanent LMP1/2 Gt2/3 class doesn’t even have to be an hour each week. I think 30 minutes races would do well as well.
Just my thoughts, but I think some might be interested.