Thank you for looking into this for me! Forgive the potentially stupid question but what is Gamemode vs Big Picture?
I'm coming from windows 10, so I'm not a Linux expert by any means. I have KDE plasma sort of replicating what windows would do, then I'm launching steam in to big picture. That setting regarding 43" scaling is only active outside a game though, it's odd. It's deactivated when on the in game overlay but activating it there doesn't fix anything.
It's not game breaking but it is for sure annoying!