ACC can be frustrating, even more so with a car like this. It's worth watching some of aris' videos to understand how the handling of these cars work, particularly gt3 aero and the brake/coast/gas method he explains. Brake: as late as possible without over doing the apex and not leaning too heavily on the abs. Coast: release brake gently into coast through apex, coast over curbs (not always), some corners require a fair bit of coasting. Gas: Getting a good run off a corner is important but getting on too early when you should be coasting means you'll probably have to get off it a bit. From there it's just a matter of being patient, not pushing too hard and the lap times start to come. I think though once you start to get to a certain point, fine tuning set ups can get you a bit more time however I'm shit at it and just use safe/agg sets depending on track and sometimes conditions.
I still sometimes get a bit frustrated when racing at rooz because I can't get "up there" but pulling back a bit and focus on the these things gets me from outside top ten to sometimes inside top ten with them, happy with that because I'm mostly on the stock sets and I'm pretty sure a lot of them spend a fair amount of time tuning.
The safe sets are tuned for understeer since it's generally the easier end of the car to control so starting out with that, especially rear engine cars, getting up to speed with the car like that first really helps me. I did the hour race Mon night with safe set and came fourth, however a whole minute behind the leaders at the end.
Not trying preach, just offering some tips that helped me get to where I am, now just gotta learn how to get the most out of set ups.