I fiddled with all the lights and found only the overhead light helps to see the switches. So, now I always keep that light on.
New users (and those who don't fly the TBM after a while) will not find its switch easily, especially in dark, and they will try the overhead light penal first, which does not help and may cause frustration.
I am not a dev, and I think the problem is with XP11. The silver-ish metallic texture always looks dark. If I knew how to change the textures and make them look brighter myself, I would've done it.
By the way, my monitor is a 29inch curved ultra-wide screen, a few months old. Honestly, I only had trouble seeing the overhead panels of this brilliant TBM900 and Rotate's MD80 (other people also complained).