Well, THR is well run and the racing there seems very competetive but fair from the few races I've experienced and they certainly run some great content and have busy grids. They are shortly running their annual 4 hour endurance with driver swaps where they encourage other leagues to join them and which I'd love to take part in but don't really have the time to commit to at present.
Compared with us, the numbers is all that is missing so yeah, don't know why that is or what we can do about it
As we don't run anything here on weekends I'll be taking part in the two THR series which begin towards the end of this month which is 90's NASCAR and 70's GP featuring the RSS car. They do also run on a Wednesday evening but that seems to be fun/test races trying out content for future series.
THR have a section in their discord where other leagues can promote their events but they seem to be mainly rF2 so not sure if they'd welcome us promoting our AC events on Wednesdays as it would clash. Maybe worth approaching the admin?