M1 Mac + External HD Game Not Starting

I have a M1 Macbook Air 2020 and due to having very little space, I download and play Steam games from a 1TB external HD.

Normally this works OK with other leagues and in Settlers besides the typical Mac issue of having to wait 2-3 minutes after launch to start the game, it ran fine. However after the Mercenaries update, it refused to launch. So I uninstalled and reinstalled and it again refused to launch. Old workarounds saying to bin it and move it to a folder in applications won't work for me because, again, I play from an external HD.

Any support? I've been playing on a borrowed PC as an alternative but I would really prefer being able to play it on my own Mac...