This time, I managed to install NFS Underground 2 on the OLED Deck. I was able to successfully get rid of the 4:3 aspect ratio and set up the PS2 controls.
Download: https://linkenc.net/EqZV9lg2ptwmOCE0D...!
PW: enjoy
Widescreen Fix: https://thirteenag.github.io/wfp#nfs (If you don't want to download my version)
Download the file.
Extract. (Ignore the Antivirus alert, it's just because the modified NO-CD exe, so allow it)
Go back to "Gaming Mode" and add the speed2.exe as non-steam game.
Go to Compatibility and choose Proton GE (latest).
Sorry, I forgot to change the upload quality from 1080p to "Full quality" in the YouTube App.