Wow thanks, I set temp to 80 and almost needed to check if the cards are still running, because they are so quite now

Can you give me a hint regarding the Core Voltage too maybe, or can I get more than these 540 and still keep the cards silent (silent != vacuum cleaner)?
You can reduce fan noise by increasing target temp: --temp-target 80, for example. No need to run the card at 70C. 80-85C is still perfectly safe and the fan will be quieter and last longer.
Well, my cards run stable with --gpu-vddc 1.125 at 1100 MHz core clock. You may need to experiment a bit to find the lowest voltage for stable operation at your desired clock.