Griff is right. 5000 series is softer than 6000 series. Two things I would try is a single flute (O flute) upcut carbide end mill and slow your feedrate down to 30 or 40 ipm. Keep your cuts light at .01 doc. Also, maybe get rid of that slot cut and try a pocket cut. If 1/4 end mill is a problem then step it down to 1/8.
I’m not speaking from experience because I’ve never cut 5000 series before. I might be full of sh%t but the above is what I would try if I was doing it.