If I had gold for every race, mission, license test, etc. then I'd likely just do Sport Mode, especially the online time trials where I typically get into silver range and thus win 1m credits almost every week. Once I've gotten gold on a race within World Circuits, I don't typically do it again, as I don't really believe in grinding - unless, that is, there's a previously-completed event that's part of the Weekly Challenges, which I'd say is another reason to keep playing.
Heck, when it comes to those events you've already done, you could try using a different car that fits the regulations, like how I used the Suzuki Jimny for the Lightweight Kei Cup race at Maggiore, and the Porsche Taycan for the Porsche Cup race at Catalunya.
Of course, even though I've gotten golds on all license tests (including the super licences) and missions, my World Circuits progress is at 89%, so I still could work on that, especially as we get more events with updates. I've also yet to start working on my last available Bonus Menu, featuring the X2019 Nations Cup, but I'm really in no rush.