I don't hear "bright" when I listen to records on glass. It seems to increase dynamics (which I value highly) and I hate felt mats. try it an see what you think.
Too many people will put a felt mat on top of another mat lowering the VTA. I guess if no mat raises the VTA, the record could get a bit sibilant and sound bright.
Like I said I haven’t tried it. And I think all glass is not created equal. The Rega sits on three 1/4” or so round rubber dots, so I suspect the glass will ring if struck.
Other tables might have the glass fully supported by a rubber mat (speculation) but in that case, the glass wouldn’t ring.
But it is a vast subject and all I wanted to point out is the VTA change can make tonal changes that might be “blamed” on the mat material, and that may not be accurate.
I actually have the Michell VTA nut for my Rega but it requires enlarging the tonearm hole. I figured if I was doing that, I might as well make a new plinth. I did make up a blank of MDF and acrylic but I scrapped it when I moved back to CT from Long Island. I had a plan to illuminate the acrylic from behind so it glowed...