OP, if you want to get faster, really the only thing I can recommend you do is practice. A lot. You may think 2500 km at the same track in the same car is a lot, but it's really not much compared to what I'm sure the top drivers have done to get to where they are.
Here's a pretty cool example of how hard the top drivers work. Take a look at the graphs. Round 3 of GT Academy was a week-long time trial taking the combined lap times of two track; one was Indy Road Course and the other was some course creator track no one had seen before. The first couple of days were spent learning the course creator track, but you can see after that, it took four or five days to drop the combined time by half a second. I was playing probably 8 hours a day at least back then hoping to shave a hundredth or two off my time by the end of a session. Some people considered me an alien back when I played GT, but it's not like I turned the game on and was the fastest out there. I literally spent thousands of hours driving, most of that doing time trials. Grinding day in and day out for hundredths or thousandths of a second helps build consistency, and that provides a solid base to experiment off of for finding more time.
It's good that you know the areas you can approve, because that means you have areas you can target. But the first thing you have to target is consistency. You need to be able to put the car in the same place lap after lap after lap before you can start making changes and seeing if they help or hurt.
If you want to get better at trail braking, you have to move out of your comfort zone and start braking later than you normally do and stay on the brakes into the corner longer than you normally do. Don't take huge leaps, do it inch by inch if you need to.
Same with getting on the throttle earlier. Pick out where you normally get on the throttle and make an effort to get on it earlier, even if it's a thousandth of a second earlier. You might find you need to change up your lines if you want to get on the gas earlier, and that's when having a consistent base will help you in knowing if the changes you make are actually helping your lap times.
Controlling the car better takes practice; there's no secret. Unless you're right foot braking instead of left foot braking and don't know you can use the throttle while braking to balance the car. In that case, that's the secret.
Slow corners are tricky because there can be multiple lines that work. The fast drivers have driven enough that they know what will work based on the track or the car or even the balance of the car. If you're not at that level, you have to experiment. And once again, having a solid base is crucial in order to compare, for example, if the better exit you get from using a late apex and sacrificing the entry is worth a gain in lap time.
I truly think the only way to improve is to drive more, and to drive with a purpose. You have to aim for consistency, first and foremost, and after that you should focus on improving on your weaknesses by pushing your boundaries. Set goals of improving your time every practice session, even if it's just a thousandth of a second. Those thousandths add up.
