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i think you missed one of the biggest factors... data acquisition.
the amount of data a pro driver collects is huge and pro teams have engineers that can collect and decipher that data into a change along with driver input to maximize speed. as a hobbiest/ amature you can dabble with this like a aim solo and taking logs when running and see what the car is doing, pulling in the pits after a few laps with no cooldown lap and have a buddy quickly take tire temps and pressures and log them, etc
also under the repeat it section adding in sim racing. it can be a highly effective training tool that although carries a higher initial cost than say going to a hpde you can get a decent setup far cheaper than a second cheap track beater. otherwise its pretty good read