I used an external monitor installing bazzite on my OG Deck. Editing the boot command to 1920x1080 worked. I couldn't read anything on the Deck screen, obviously, lol, but the monitor worked fine.
But doing the same doesn't work for the OLED Deck. I set it to 1920x1080 and changed the tag from :39 to :testing, and when the installer comes up, it's spread out across both screens and completely unusable, lol.