Dunno. I don't run 24 hours a day personally. Don't see the benefit to harvest a couple days earlier? And running 18 hours a day has NEVER affected my yields. So there's that. You could reduce the schedule to 18/6 for a bit. Shouldn't hurt anything. I would stick to that schedule for the remainder of time though and not raise it. I once changed my schedule to 16/8 once to see if I could prolong harvesting for a week when it was really hot once. I dunno if it helped, I waited the week anyway and it was all good.
In addition to the light thing- which is a total shot in the dark. Only thing I can think of is possibly trying to lower the temps? Like a good amount. Like a 10 degree drop in temp. I would think that getting them colder, would delay a lot of things. YMMV.