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.