Currently there is no reason to drive more than the Required Goal amount of laps in Practice, unless one wants to level up their car. There is zero reward for spending additional 5 or even 10 minutes still lapping that circuit before the race. The Practice payout after a race is always the same as the Race payout itself. For this reason, I think payouts should increase proportionally to the amount of time spent in Practice.
You drove the car for extra 5 minutes? You get 50% extra Credits for both Practice and the Race. You drove all 3 laps and 10 minutes? You get double the Credits for both Practice and the Race. You did all that and won the race? You get triple the Credits. It would finally be a reason for me to always complete the full Practice.
Of course, the biggest tracks like Le Mans often have decreased Extra Practice time, down to 5 minutes, so the payout should also be scaled for that.