So, I've done it.
I found a rear bumper from a wrecked 2021 mirage in the exact same color as my mirage.
I tried fitting it today and it fits perfectly:

So now I've got to fix the little damage spot in the top right, find some reflectors (€28.50 a piece from mitsubishi themselves, I have not been able to find them cheaper) and most importantly, fix the license plate lights.
Those lights are a good 10-15cm further back in the old model, so I guess the new ones have some extra pieces there?
Also, does anyone know if the 2020 & up mirage has a rear bumper bar?
My guess is yes, since it should fit very easily, but I cant find pictures or anything.