That’s funny - I have the same router, the same CyberTech tach and got the SAME numbers. I put sharpie mark on my router housing too. I also put a mark on the dial at 18k rpms so I could have a consistent speed for aluminum. I had to color my collet nut with a sharpie to blacken it up so the reflective tape could do its job.
Well I only tested it in aluminum and brass at .005" DOC and 50 IPM. So, there wasn’t much load on it but it stayed rock steady. My giant Hitachi stays at the same RPM no matter what I am cutting, and that is 7 or so years old, so I assume the speed control on the Makita is as good or better than that.