Car design have a pretty big effect most people do the following:
Give up one season and focus fully on next in terms of design, if your current car is around 30% this allows you to update like 4-5 car parts per day depending on the amount of designers you have. Also if you train 'design' after each race you get an added 1% onto each part. You can then start next season with like 70% car. But the downfall to this is that next season you won't be so great. Due to the car being a higher % you can only upgrade 2-3 parts per day which will set you back from others.
Ps: make sure to push hard in qualy and don't go for 1 stoppers because they are not faster (depending on race length) 2 stop at Australia with like 30laps and 2 stop softs will dominate any other race strategy. I tend to run a 2 stop softs on all tracks that are NOT 'very high' tyre wear
