Is there a way to reduce the frequency of "Full Course Yellow" flags (which go on forever) in an offline race?
Yes, adjust "Safety Car Thresh" value. Above 1 will make it less frequent. There are multiple values, set them all
Your better off just disabling full course yellows. rF2 has known bugs regarding several pieces of the FCY process. The cars do not stay in formation, get out of order, which causes large gaps to form in the pack. Then, as you noted, the FCY goes on for a long time. IT WILL NOT BE FIXED ANYTIME SOON. Just turn off the flags and you'll be happier. Don't ask....offline is not a priority. Any work in solo play will only come after the UI, Comp system, and DLC projects are complete.
I have a bug where any yellow flag causes the AI to slam on their brakes and stop on the side of the track? The black flag only option doesn't help?