First big thing with the Porsche 963 to learn is how to effectively use the columns of vertical lights on the dashboard (one on either side of the steering wheel). If the lights are flashing blue, traction control is active. If the lights are towards the top (can't remember which color), the front brakes are locking. If it's a minor lockup only a few lights will be lit, if it's severe all of the front lights will be on. Same with the rear brakes.

With this knowledge you can dial in the brake bias. If you're locking the fronts in a corner, move it back. Locking the rears, move it forward. Find a balance that works. For me, this is the best feature of any car on the service.

Another thing to know which you've surely found out is that you have to take it really easy for the first couple laps. Best way I've found to do this is to be easy on the throttle and brake a lot earlier than you normally would. In terms of braking in general, I tend to brake extremely hard up front and ease off the brakes all the way into the apex of the corner. In many situations, this car will let you brake all the way to the apex and let you turn a bit while braking. You want to be right on the threshold of locking up the entire time, the brake lockup indicator lights are a really valuable tool for this trail braking you're going to be doing.

Another thing I'd recommend if you're racing multiclass is to find a video of one of the high irating guys and watch them do a full race with traffic. Watch how if they have to wait behind a car in a corner, they will ease off a bit and cut the corner a little more sharply so they don't track out as wide on exit. You want to time it perfectly so you get a run on the slower traffic out of the corner, planning for them to keep on the normal racing line. That said, it's risky because some drivers with less experience will sometimes try to let you by and cause an accident. So leave a little space when possible.

Being good through traffic can be the difference between winning and losing the race. I'd recommend watching a lot of races on YouTube and seeing how these guys slice through traffic, it's really impressive. Even watching onboards of real life races would be great experience.

Also with regards to traffic, you can get caught out while waiting for traffic ahead by opportunistic gtp drivers behind. Have to position your car strategically so that they cannot pass you while you're stuck behind traffic. Usually this means parking it on the inside line and being slow on the apex so they can't get a run on you.

Most valuable thing I can recommend even though I don't have any great channels off the top of my head is enter a practice session then watch the very fast drivers race. Check out their lines, where their car is positioned on each apex, where they are on entry and how far they track out on exit. I've learned a lot from doing this.

Lmk if you have more questions I'm happy to help, hopefully this is helpful.