Japt v2.0a0, 30 29 28 bytes

Takes input in lowercase, prepend v<space> for +2 bytes if that's not allowed.

óÍ®n îZèÍg"- ="òìyÈû4 Ôû6'_

Try it or run all test cases

I'll have to come back later and add a separate section explaining how the partition works as it's a difficult one - I'm not sure I fully understand it myself yet! I'll also add some more detail on why Í="n".

óÍ®n îZèÍg"- ="òìyÈû4 Ôû6'_     :Implicit input of string
óÍ                               :Partition into the groups we need
  ®                              :Map each Z
   n                             :   Convert to integer (the letter at the end is ignored)
     î                           :   Repeat and slice to that length
      Zè                         :     Count the occurrences in Z of
        Í                        :     "n"
         g                       :     Index into
          "- ="ò                 :     Partitions of "- =" of length 2
                Ã                :End map
                 ¬               :Join
                  y              :Transpose
                   È             :Pass each row through the following function and transpose back
                    û4           :   Centre pad with spaces to length 4
                       Ô         :   Reverse ('cause centre padding is right-biased)
                        û6'_     :   Centre pad with "_" to length 6