Just my two cents:
Suspension: it has to do with the responsiveness/grip ratio. Also affects tyre wear (supposedly; more on this later).
The more stiffer (higher values) the more responsiveness at the cost of grip and more tyre wear.
High responsiveness is usually good in tracks with a lot of small turns like Singapore and Monaco (although Monaco, due to its special - ie bumpy - features, may take benefit from softer suspension) where you brake, turn just a short time and then throttle again.
Grip is usually good in tracks with a lot of long turns, like Malaysia (turns 5-6, turns 7-8, turns 12-13) and China (turns 1-2, turns 7-8 and turns 12-13) where you are turning a long time. You can go with softer suspension to get grip and avoid higher aerodynamics (that reduce top speed).
You must find a good compromise between responsiveness and grip anyway because in almost every track are short slow and long turns. And even long turns are also combined (as you see above) and you also need some responsiveness to get the car changing direction quite fast without losing to much grip.
Toe: it affects responsiveness exclusively, AFAIK.
Values to the right mean higher responsiveness at the cost of instabitly when you drive in the straights. Changes in this setup are far more noticeable when you're using steering wheel without deadzone and near to zero linearity.
I completely agree with you in the aero part. It's almost useless in this game. You can get grip in fast turns without losing top speed thanks to AR bars, suspension and camber. I don't see the point in this game to have high aero (except maybe under wet condition?), since changes in AR, suspension and camber (at least in my case) don't make huge differences in tyre wearing terms, hardly one extra lap in my experience.
If they only made tyres more susceptible to AR, suspension and camber changes, we should be able to use aero to balance grip and tyre wear.
