This is what I know from my experiences, some maybe relevant, some may not, however in the end, it's for you to choose what is best for you.
I use the slow corner fix and true AI, I haven't tried the RDDEV mod, perhaps I should, I digress. However I started playing with all assists on and have slowly turned them off over time. Soon as I am reasonably comfortable with what I have got used to, I switch another assist off.
I have not played too much with Fuel and Tyre sim on, however I have noticed that as I turned off assists my general lap times improved. With TC on medium, race line on, tyres & fuel sim off and all other assists off, I can generally qualify in pole for races with legendary AI (I customized the settings for legendary AI, to turn Fuel & tyre sim off, also switch flag & rules to min.)
Braking assist should be off in all cases, it manually breaks for you proportional to the colour of the race line, the further the shade from green it is, the more it will break. I tend to start braking when the line race line is red, so this option is never going to work for me.
ABS should probably be off, as if you start to spin, you can tap the brakes or gear down and it will correct your spin, with ABS on, you just continue to spin off the track and lose much more time (5+ seconds compared to 1s)
As for car setup, each person has their own driving style, so no matter how good the laptime is for a setup that is posted for a track in the setup section, the individual's driving style will negate how good the setup is to some degree. I generally use Andrew Bortz, Viktor Andersson or Bram's setups as a starting point and have tweaked these to suite my style, for most tracks, in most cases, I have changed something to suite me though.
I have put a reasonable amount of effort into developing car setups, I created a spreadsheet with setups I like from the setup forum, and added my own with tweaks of this and that. I spent about a day initially, doing each track to find a reasonable setup and saving it, then a further day tweaking each one at a later date.
You will find that with the setups posted, that they are somewhat car specific, in general, online quick races tend to base the car on something very much like the Redbull, however for proper races on or offline, each car has it's own characteristics, the most noticeable is the gear ratios, for example, a Redbull has different gear ratios than a Mclaren. From what I can gather, Codemasters displays this information rather misleadingly, so when changing team's, it transfers the ratio's, but not the top speed, so the gear setup for team A will not mean anything for team B setup.
Comparisons:
Lap times of a setup I'm happy with to any of the generic garage setups, I will gain at least 4 seconds per lap on most tracks.
For each assist I switch off, I roughly gain about 1s per lap.
Currently, I am learning TC off, however, when I don't play for a week, I lose touch and end up spinning off track about 1 in 3 laps, so sometimes I switch TC on to medium to start off with. This is what you would call a bad habit, that I have learned to drive with TC on, so I tend to revert back to driving in this way.
By far the biggest factors for improving my lap times are
1. Practice
2. Setup
3. Assists.
Although there seems to be some cross over between which factor has the most influence, between setup or assists, as I've accidentally driven online races with a bad setup, as in really bad and still managed to drive faster than people of comparable skill level to myself, probably only due to the fact that I knew it was bad, so focused on driving faster and closer to the edge of my ability and after all, if your not driving on the edge, you're taking up too much room!
I hope that this is helpful, although maybe a little long.
