I was wondering if there is a way to run the circuits without any opponents, as a practice or timetrial mode, so you can spend some time learning the corners and braking points?
You can, Timetrial is called Rivals in this game for some reason.
You can either use Rivals or make you own blueprint and in settings set number of AI to 0.
Go to whatever location you want and enter the race. You will see 5 squares: Solo, Rivals, Create route, Co-op, PVP.
If you go to Rivals you can choose what class you want and basically you are ready to go. If you have friends that have time in that track you will chase their ghost, but if you want you can change your opponent. When you beat your opponent time click “continue lapping” and do more laps until you are happy.
If you want to create a blueprint you need to click Solo → Create blueprint → later choose car options → and when you will have screen with “Test” “Publish” you need to go to “event settings” and there will be option to change number of AI.
Your welcome 
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Awesome. I just assumed Rivals meant online head-to-head, never even tried it. Thanks for helping my lazy self out