One of the most annoying and time intensive things are that I do not know where the track limits are. Using the white line as indicator does not work, because sometimes you still get off track (road america) and I cant test every corner 20 times for this purpose. And if you do not test it, you are easily 1-2 sec off the pace. It seems you do not change them often. What I would like to have is either something that is consistent which will not work out, I think we all know it or pictures of the track from top down view and a line where the track limit is. Before I go to a track, I can then see limits. We have something similar in real life in driver briefings before the week end. I can't be the only one who finds it extremely annoying and even off putting to learn new tracks.
The general rule is (if still up to date) that two tires have to remain in contact with the track and kerbs are considered to be part of the track. On corner exits that seems to be applied fairly consistently (although there are still exceptions), on apex kerbs its a lot less consistent but its still a starting point at least
The thread where this rule was stated, is not accessible anymore, and I interpret that as if it's not applied anymore. There were numerous cases that track limits changed since then (often without any mentions in patch notes). The current state of track limits is criticized a lot and apparently won't change (apart from fixing some obvious outliers here and there) because there's a new track limit/cut detection system in development.