Yay! After 2 hours of messing about with rF2, I can now join the RD server again!
In case other people are still having issues, here are some things to try while they're the top of my mind. Some of the steps might be redundant, and someone with more rF2 experience might have some better ideas that aren't as overkill as this, but it worked for me.
Look in the follwing 3 folders for any mods that build on top of any of the cars or tracks that this race will use:
\common\rFactor 2\Installed
\common\rFactor 2\Packages
\workshop\content\365960
The installled folder should correlate with what you see when you go into rFactor 2's content page on the new UI. If you see any modded content in the content page that's marked as locally installed, you can then cut and paste it out of the installed folder (can always paste it back if things go wrong). Once you've done this for all of the content, restart rF2 and they either shouldn't be there anymore, or have the uninstall button clickable (if so, click it and it should disappear).
The packages folder should be empty except for the skins folder, so move anything you see there into a backup folder.
The workshop content folders are full of all the bits of content that get downloaded off the steam workshop. It's probably easiest to search here, either for the car names that RD are using or for any mod names that you know you use that sit on top of the cars that RD are using. Remove any 3rd party modded content from here, and unsubscribe from their respective workshop pages through your browser.
Also make sure to delete / backup any RD-specific files, can always download them from the server again later.
All of that is with the intention of making your rF2 installation as close to fresh as possible. Now try to join the server again, and pray to the rF2 gods.
If all that fails, then try closing steam, unsubscribing from the Indianapolis track on the workshop (through your browser), opening steam and rF2, then closing both fully again, subscribing to the track again through your browser and then opening steam again, waiting for the track to download and opening rF2.
If that doesn't do it, I don't know what will
anywa,y, I'd better go and actually practice now hahaha