Hello! I have been working on this issue for a while and I can't seem to figure it out. I'm new to the CNC scene so this issue might be a walk in the park for an experienced person. Any help is appreciated

I am trying to profile simple shapes to fit a silicone strip in the cut gap.
the cut width is supposed to be 8mm with a depth of 3mm, but for some reason, the width varies between 7-7.8mm and reaches depths of 3.5-4mm (as presented in the octagon cut in the attached photo). The material I am using is a 6mm thick sheet of acrylic.
the same issue occurs even when I'm using a much more forgiving material such as MDF.
I am cutting with a 6mm o-flute (1/4"). I have attached the feeds and speeds at the bottom
the machine is a 750x750 WorkBee desktop CNC, Nema23 2.8A motors, TB6600 stepper drivers, Arduino Uno, 12V 15A power supply, GRBL engine, Mekita RT0700C router
i am using Cut2D

I have tried:
- slowing down the cut
- using fresh endmills: seems to make a somewhat accurate cut for a short distance then the issue starts again
- switched from a 3D printed spindle holder to a solid aluminium one
- tightening the plastic axis wheels: they are very tight, is that a bad thing? there are also track marks on them I'm not sure if that's normal

I really need help on this one