Hi! I didn't get anything similar on internet, except for a guide now kinda lost, that describes this very easy way to get to 1 million Lums by exploiting the save file.

N.B.
I usually play games the intended way and earn achievements through regular gameplay. However, in this case, I fell like an exception can be made. Ubisoft discontinued the online challenges for this game, making a this achievement effectively impossible to unlock through normal means, without spending tedious repetition of boring steps, and without spending tons of hours. I decided to modify my save file not to cheat the experience, but to reclaim what was once a legitimate goal.

This decision wasn't made lightly. I respect the spirit of achievement hunting, but I also believe it's unfair to penalize dedicated players just because a publisher pulled support for a feature that was once required. My goal was simply to restore access to content that should still be attainable, had the original systems remained in place.