The Rotor RPM in a chopper is stable and mostly not touched much. When you raise the collective, your rotorblades are changing angle. That's how collective works. It's not changing any RPM.

It's actually correct and incorrect !
Real helos, raise or lower rpm based on how much collective you raise.

Small adjustments are usually dealt by increasing rpms, heavy collective adjustments, instead, are dealt both by rpm increase and, in the same time, rotor blades angle increase that will lead to a subsequent rpm decrease.

That's also why when you hear a helicopter stressing the engines (like airshows, are perfect examples) you hear a slower "flap-flap-flap-flap" while doing manouvers.

This is just an observation by a non-real life helicopter pilot by reading around how a helicopter works

edit: another good "hands-on" example is playing DCS Black Shark

Most of the instruments such as torque gauge, temp gauge and fuel gauge aren't functional. Its just static images as it seems. Hope this will be different in the full version!

Collective refers to the (collective) pitching of the blades. (All of the blades, hence "collective".) As you pitch them up so they add lift, they will also meet increased air resistance, and thus slow down to a lower rpm.

The engines will normally not be working at full capacity, because a governor caps the rpm. So the engine output will then be automatically increased to try to keep the rpm constant, but it shouldn't be a surprise if this lags a bit.

(In my observations, I've noticed real helicopters being the most noisy when hovering; presumably related to the aerodynamics of a rotor blade in the wake of another.)

When hovering they put a higher pitch rate to the rotorbaldes because they need more lifting force, as opposed to when flying, where the rotor becomes very similar to a wing

I'm a real life CH47 Chinook pilot.

In a helicopter, Rotor RPM remains constant in normal cases. It CAN decrease a LITTLE bit when pulling power/collective. What happens is the angle of attack of the blades increase, they'll have more air to 'push down', this requires more power, the 'governor' will react by increasing fuel flow, but there can be a little bit of lag so the RPM drops just a little bit, but will quickly return to 100%.
Same goes for decreasing collective, if you decrease rapidly the rotor system may disengage from the driving train and rotor RPM may increase.

this seems about right.. pulling the collective should give the rotor blades more resistance due to a higher angle, thus decreasing engine rpm. the engine will however counter this so the low rpm shouldnt last long.

So it seems that the issue here is mainly the gauge reading zero when they are clearly still rotating (powered flight) as OP says

Of course don't expect a perfect simulation of the aircraft but at least have gauges make sense and not show total BS

Instruments in helicopters are still work in progress. We thank you for your feedback.