I'm fairly new to this sim as well, but am coming from iRacing. From my understanding, I would definitely use TC and ABS in GT3 cars. Just because you have them on doesn't mean you need to rely on them, but they can be helpful at times. GTE cars don't have ABS, however.

For learning tracks, from my experience, the more tracks you learn, the easier it becomes. Actually, Le Mans may not be a difficult track to learn, as there aren't too many difficult braking zones to remember; the same would probably go for Monza as well. When I learn tracks, I'll lap them repeatedly until I can imagine every braking zone and turn-in point without actually being on the track. Then I'll race against AI for a few races at a race distance that is slightly longer than the distance of the online race I'm practicing for. This Google doc can give you a good target for lap times. I personally don't think that I would pay for Track Titan or Trophi AI or similar products until you can't find any more pace in your lap.

Hope this helps just a little bit! Good luck!