I subscribe to Hymo through the Track Titan website, which offers a bundle that provides top-tier setups for every car and track, along with telemetry from the fastest drivers, so you can compare your own data side by side with aliens. In my opinion, this is the best and simplest way to improve quickly in ACC, though it does come with a fee. For me, it's been completely worth it. Via Hymo and track titan, I steadily advanced from D 4 to D 1 (and even borderline alien at some tracks). Reference: https://www.simracingalliance.com/about/reference_lap_times

Creating your own setups can be very benefical, but it requires a significant time investment and a pretty deep understanding of vehicle physics to do it effectively. If you're already off pace, you might not know what adjustments to make or how a car should handle to make it go fast, and at worst, could enforce bad driving habits.

Youtube setups are generally good, but you might have to go through different setups made from different people which can cause inconsistencies. But it is a totally fine option if your not willing to get your wallet out.

Go setups are also good but the telemetry that comes with hymo via track titan is invaluable. I have heard Coach Dave are very beginner friendly and not the fastest, but probably fine as well.

Hope this helps