Bad toolpath and/or cheap bit can damage quickly/unexpectedly. Consider verifying using another new bit, and/or visually inspecting cutting surface of the carbide single flute upper cut bit guesting you’re using (e.g. cheap $20-$30 USB camera microscope are more useful than I expected).
With enough decent Loctite, and patience to wait for curing, should be possible to stop the Hex head M8 bolts from spinning. How are the Nylock M8 nuts?
If you’d like extra eyes/thoughts on slicer settings, then consider sharing filament type, Nozzle size, temperature, layer width, layer height, number of walls, using alternative layer infill wall, etc…
I mentioned verifying the router bit because we’ve seen people like Philipp print multiple Cores, when root cause was dull bit (or too many flutes). E.g. Can you cut this please? Takes 34 seconds. :)