With my previous EQ-5 I reduced the maximum slew rate in ASCOM/EQMOD as well. That mount was on the small size for a 200 Newton, so driving it at full speed yielded similar noise problems.
Upon your recommendation I did a test this morning with a lower slew rate in ASCOM (540 instead of the maximum 800). Noise goes indeed considerably down, but the noise from the mount is notatbly different than from before. I also ran the mount at max and lower slow speeds without the scope and counterweights and that seems to have a small improvement (logical since there is less inertia to push around by the motors).
It seems to be a combination on how the stepper motors are driven in 3.07 and the resonances in the system (so may also be a bit dependent on the scope used). In the change log it is stated that since 3.04 the current at low speeds was reduced. This is obviously done to save the batteries in the field, but since I use the EQ6-R solely around the house, I use a wall outlet. So less relevant for me.
However, based on the suggestion above, I managed to download the older firmware driver from the SkyWatcher site: https://inter-static.skywatcher.com/downloads/mc015_firmware_0303.zip (thanks for the suggestion, sometimes the solution is more simple than you think!)
I have successfully installed the iss. 3.03 firmware, and the mount is now running back in its original super-silent mode! It was the low noise levels, which really impressed me during the unboxing process (and which I like to show off to fellow backyard astronomers in the area). For your info: on the SynScan I am running the most recent 4.39.10 firmware.