I would not be comfortable running the fan that slow. The cooling solution isn't exactly overpowered on these cards.

Also the PCI slot coolers suck air out of the case so would counter-act the fan on the graphics card.

My active Geforce 2 Ti has a cheap cooler from Aliexpress, similar to this one: https://www.aliexpress.com/item/4000290464402.html?

The fan is running at ~9v to keep it quiet.

No, i don't mean these PCI coolers which transfers the air out of the case.

I was thinking more this solution, which has a good airflow accross the whole VGA card.
I assume, maybe this would be possible to help the VGA temps down including an acceptable noise...

The problem as I see it is that only the GPU really needs any cooling. So you are "wasting" a lot of airflow (and noise) with that cooler. On the plus side, if you get a big heat sink of some kind you can run it without a fan 😀

I would measure the temperature of the backside of the GPU with the default cooling and then measure it again after modifications too see if there is a difference. Maybe looping something like 3DMark2000.

Not totally accurate but it should give you a general idea of the cooling performance. You can use a thermocouple so you can close the case, but an IR temperature gun would probably be fine.