Raise the material up another 2″. With aluminum rigidity is key.
To give you an idea, most other small mill only have a max travel of 3″. Raising it up should help a lot. Every single mm counts, Get your bit as far into the collet as you can, raise the tool all the way until the bottom of the z tubes are abut to come out of the lower bearings, mount your spindle as high as posible, the raise your material all the way up.
As for the holding tabs, you could be cutting so shallow that the aluminum is actually bending under the bit. Raise everything up and try to get depth of cut a little deeper. Which would also help cooling, you need to get some chips out of there they carry all the heat with them.