Hello,
First of all, you should remove you custom code that broke your Checkout page. In general, to make the “Checkout” page full width you need to use our theme presets. Please navigate to Theme Settings -> Theme presets -> create a preset with the condition as in the below screenshot.
https://prnt.sc/lyUztzxRygvu
Then edit Theme Settings of this preset and navigate to General -> Layout -> for the “Site width” set the “Content full width” option
https://prnt.sc/6fwvCk35sxJ2
The result will be as on this screenshot: https://prnt.sc/g4i3X9QqbLUB
Kind Regards