That was all done in the last 3-4ish days. Today was relatively less nerve-wracking than that - at least it didn't involve me scrapping a Unity project cause of code frustrations. I focused mainly on getting a nicely working Checkpoint and Lap system in place - I think my current goal for considering myself at least 'done' is 3 levels with a time trial setup.
That said, here's my Checkpoint system:
Here's some views of my checkpoints - basically having colliders as triggers and an animating flag with a border around it (done using a circle sprite and circle sprite mask, easier than I expected) for iconography. I also decided to have the user see what's the next checkpoint to go to via having it slowly fade in based on how close you are.
I think it looks pretty good - save for the instant going away once you touch the collider. I'll probably change it to turning Yellow until you finish a lap, but that might not be tonight. Tomorrow will likely be that + setting up lap times and total time.
As for theme, I'm thinking instead of a future racer, I'm going to make it a submarine racer. The main mechanic being toying with being above water vs underwater, and the advantages/disadvantages those can give. Things I'm thinking about are like moss or weeds on the water surface you can dive under for shortcuts, but will slow you down.