Revision 128ba722-3d66-4692-97c6-df61dffc6ad3 - Code Golf Stack Exchange

#Mathematica, 129 bytes 

 Max[(s=#;r=#2;Max[Tr/@Partition[#,r,1]&/@Join[s,Diagonal[s,#]&/@Range[-(t=Length@s[[1]])+2,t-1]]])&@@@{{#,#2},{Transpose@#,#2}}]& 
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