The screens are actually the very reason I choose a thinkpad over a mac. Gloss screens are pretty much impossible to work on if you prefer dark colorschemes, and at least the thinkpads give you the matte option.
I don't think I fully understand what conditions you are talking about. I very rarely put my screen brightness any higher than 50%, and I've never maxed it out. My first thought is that it's not the brightness, but the contrast, that is the cause of your problems? Or perhaps I just have a model with a brighter screen... (currently on a T460, though I don't remember the screen specifics)
EDIT: I might add as well, that part of the point of using a dark colortheme, is to not be looking into a lamp...so perhaps I am simply more okay with lower brightness, because I explicitly look for dark colorthemes. That does mean I am very sensitive to finding a colortheme with good contrast.
Do you think it was as good as a factory-produced matte screen? While I was aware you can do this, I'm a little hesitant to do something which is an absolute minimum for usability, in case I'm not happy with the outcome. It feels a bit like having to install a keyboard on the laptop myself, because the laptop didn't come with one...
I honestly couldn't tell the difference. You lose some output and viewing angle, but you did so on the matte screen as well. I had it on for like 6 years and my wife still has hers on from 2011.
I like matte screens too but my problem is their brightness. T495s comes with a 250 nits screen which can be quite dim. The Intel models have more screen options.
But dim screens just don't work, no matter if it's matte or not. Bright screen with anti-reflective layer work much better, hate to say that.