Are you tired of black bars on your ultrawide monitor? Are you still looking for a mod that fixes all the UI issues? Come no further, this is an ultrawide mod I found online that will fix all issues all other methods had and instead create a much easier and faster way to play Sonic Racing Crossworlds in any and all ultrawide resolutions.
I am not the mod creator so all feedback should go to them.

Go to https://codeberg.org/Lyall/CrossWorldsFix/releases
Download the file marked CrossWorldsFix_0.0.1.zip (or the latest version) from the "Downloads" section on the website.
Extract the contents of the release zip in to the game's binaries folder. (e.g. "steamapps\common\SonicRacingCrossWorlds\UNION\Binaries\Win64" for Steam)
Here is an image of what it should look like if it helps.
And you're done!

To install via Linux & Steam Deck running Linux for 16:10, make sure you do all previous steps.
You do not need to do this if you are using windows!
Add this to your game's steam properties by right clicking on your game (Properties > General):

Open CrossWorldsFix.ini to adjust settings.

SpanRaceHud will let you scale your HUD to the edge of the screen.
SpanRaceHUD = true
SpanRaceHUD = false

No. This person has a whole backcatalog of stuff they've contributed for different games and everything here is all open source.
From what i've personally tested, No. It's all client side.
Currently, No. But I will update this guide once support is added. In the meantime you could try the hex code editor method: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3575546411
Make sure you placed the files in the Win64 folder in your game directory. (Refer to the file path above)
If you're looking for an ultrawide mod for the switch version, I can point you to here https://gamebanana.com/mods/623949
Most likely! The file format is the same so there's no harm in trying.

I hope this helped anyone looking! If this did help you please consider giving an award <3