Pushing a XM-L on a 16mm aluminum pad driving it at 2100ma, will this work ok? I know you can push these 3000ma+ but not using a copper MCPCB, I don’t want it to prematurely blow.
Many budget lights sported XM-L emitters @ 2-2.8 A on simple aluminium pads in the past, so yes it will work.
Many tube style lights like UF 2100 and Xiaozhi for example.
It’ll be fine.
Up to 3.8A is just fine with an XM-L on aluminum, although not as efficient at that level as on copper. 3A is just fine with an XM-L on aluminum, all of the XinTDs and many other lights were and are running at 3A with no issues at all.
A general rule of thumb is that it's safe to run a Cree emitter up to its rated drive current on aluminum for thousands of hours of service. Copper (direct thermal path) helps us drive them much further than "maximum." 
IIRC Cree's designated maximum for the XM-L2 is 3A.
Great, thanks for the help