I finally finished fixing this shirt. It took about two seconds so I don't know why I didn't finish it earlier!
I simply changed the buttons (because the original ones were coming out of their backing).
And I took it in 1.5 centimeters on each side, including the sleeves. It looked like a sack (in cute fabric) before and now it looks a little more shapely:
Thanks to T. for taking the photos! That makes one more thing I can wear during the remaining of Me-Made-June.