Well, good news bad news.

Good news is that @Panzerknacker solution worked for starting the game in the proper resolution under Gaming Mode. Thank you very much!

Bad news is that the game doesn't detect, or at least readjust, if the resolution changes by connecting/disconnecting an external ...

Mm, just tried that and it didn’t seem to help. Whatever resolution you start with you’re stuck with, and it seems to start with 800p no matter what in Gaming Mode.

My apologies if this isn't the correct place, i wasn't sure if it was a bug or just a niche situation that was simply overlooked.

On the Steam Deck using an external HDMI display:
If you start Factorio while in Desktop Mode the game will run at your native resolution. However, if you start the game ...