One should be able to win the FGTC with any Formula car, even the F090/s...you'd need to use drafting to win the Test Course race with that car, but you should be able to lap the field in all the other races.
Tire wear is really not an issue. Even though you'll pit more frequently than the AI, the fact is they drive terrible racing lines and you should be well ahead of them by the time you have to pit.
As far as the downforce, maximize the front everywhere, except the Test Course, and back off the rear everywhere just a bit...max downforce on the rear doesn't gain that much grip and only increases tire wear on the rear tires.
Don't slam the ride height to the lowest setting...the suspensions on these cars are very good, but give them some "play" to take advantage of the dynamics.
Once you have a suspension setting that works well for your driving style, leave it alone (except for the Test Course) and adjust only the transmission for each track. Côte d'Azur will be a very short tranny setting, but the rest should be fairly similar with just some minor changes to the Final drive.
For the Test Course...
Minimize downforce
Set Camber to 0/0
Toe to 0/0
If you run the F090/s, stretch your tranny so that you won't redline while getting a draft assist. If you pop into the front before the last turn, you should actually start to slightly drop speed in 6th gear.
If your driving skills are up to it, you can sweep this series with the R390 and GT-One. You can win the series with the 787b, but won't beat the AI at Seattle or GVS...2nd is the best the 787b can do at those courses.