You said ISO25k.
That was either a Typo or you're wasting the "Faster glass".
Any way I have sang in a rock band for 2 years paid about between £40-100 per member so I know dives a lot of dives.
I also specialised in gig photography for about 5 years now.
I have a portable lighting set up in case there isn't a lighting guy in said dives like I said before, in the image link posted below (an off cut from a shoot)you can see the Soft box on the floor, next to the smoke machine, so that the smoke is caught in the light.
As for clubs, there really is no excuse to be lazy with your lighting, you are a photographer of course a venue doesn't want flash get a steady fold away kit turn up early and set up with the band.
Also reason I came up with that lens isn't to say that's the lens you used, those are the settings you had on the lens you used, would have been equally used on a cheaper lens for less effort.
Perfect example of a lens capable of that is the kit lens on a lot of the images you had up.
Gigs are grotty places and if that lens is an expensive one, which it probably is compared to the kit lens for all of 60 quid getting smoke and detritus in it is bad.
As for full frame, I was trained on standard SLR cameras and have darkroom training, 3 years from A level to Degree. We were told you get higher grades for self developed photos so that's what I worked with, until about my second to last year when I made the Digital switch due to work load and lack of time to get in a dark room for a few hours.
Full frame does let in more light though so why that would be a down side I don't know. But yes I do shoot on crop, I prefer it for ranged shots and reverse docked macro shooting.
I the dark smoky filled room... I have posted it for you here.
[link]It shows the lightin set up, not the best of the day not even in final output but the smoke is clear and at the right hand side I took another and that was better but for the purposes of this post that suffices.
Now I have never shot at 3200+,
Because if a job won't pay me, I don't turn up.
If they don't let me have my say, my camera and kit go home and the client covers the cost of travel pre-booking including site recon.
It's all marked out no fuss no muss.
For both my band and my Photography
Yes faster glass is nice to have, yes they let in more light, and yes they are better for output, but it's an expensive paper weight when you go to lets say a rock gig, in a small pub where people spill beer everywhere and your being paid next to nothing.
One drop of juice in the lens, smoke sucked in, a knock or a bump by a punter, good bye nearly 1k,
Cheap steady lighting kit 90 quid 2 lights portable,
Kit lens 60 quid.
Job done. Someone writes them off no insurance premiums need to be raised.
Just shell out the peanuts to buy more.
How bad would it be if you tripped over, this bad?
[link]Seriously even with insurance getting a hit like that will make your heart sink.
At least body's are durable - no lenses are air tight.