Hi Zenjungian, To try and answer simply.. I would say 'Practice makes perfect'
I suggest before taking to a race that you run the event qualifiers for a good hour plus to try and find your optimal lines, keep pushing trying to beat your best lap until you cant! then you can race as you have found the best speed you have for now
Running Manual gears is always going to be faster than Automatic due to the fact that you can precisely use the needed gear instead of letting the game decide for you.. It makes up heaps of time alone per lap! Assists are a good option to let the people who aren't so seasoned get a feel of the thrill of racing online without spinning out all the time and are not something to be ashamed of using, as you get faster you will find more confidence to turn the assists off 👍
You could also watch some of the top laptimes if they are available on the leaderboards in game to see where those drivers make time and the lines they use.. I know there's also bundles of replays available on youtube for numerous GTSport combo's from Wheel users and DPAD drivers also that make great reference for people to learn from.. A bit like riding a bike with stabilizers! when you get the just of it the stabs will come off
Do whatever set up adjustment and use any assists that makes you feel consistent and confident when driving on track... that is the key to racing and enjoying it.
Good luck mate!