Not very thick, a double glass I think about 8 mm…
As you mentioned is not very significant as is behind the speakers so highs and mids don’t be reflected so much.
Probably more influence of a marble table just beside the left speaker is reflecting more highs and mids than the glass
I have seen places that had some glass walls that were made form hunks of glass the size of cinderblocks.
The particular one that I am thinking of was a restaurant named The Cavallaro in Charleston, SC that I frequented about 4 times a year with dates when I was a teenager in the 1970's.
That takes me off subject a bit:
It was a higher class but accessible place if you could become passable in a coat & tie (despite my longer than average [for their clientele] hair.
And the girls that I dated always new how to dress well & sultry without being indiscrete.
Ah, an epiphany: perhaps that is why I did not get married until I was 48.
At any rate, the entry doors where between these glass blocks.
And I have seen them in one other place but cannot recall where.
The place had pretty good acoustics inside, as there where a number of softer surfaces inside, even though there was usually some sort of somewhat subdued band (Jazz, 40's Swing & Shag music).
& in between sets someone played piano, usually.