05AB1E, 19 17 bytes

áΣl}.γl}éεgyθ‚}JJ

Try it online! or as a Test Suite

Explanation

á                  # keep only letters in input
 Σl}               # sort by lower-case
    .γl}           # group by lower-case
        é          # sort by length (stable)
         ε    }    # map each to
          g        # its length
             ‚     # paired with
           yθ      # the last letter
               JJ  # join to string

05AB1E, 19 17 16 bytes

Saved 1 byte thanks to Kevin Cruijssen

áΣl}.γl}éεgyθJ}J

Try it online! or as a Test Suite

Explanation

á                  # keep only letters in input
 Σl}               # sort by lower-case
    .γl}           # group by lower-case
        é          # sort by length (stable)
         ε    }    # map each to
          g        # its length
             J     # joined with
           yθ      # the last letter
               J   # join to string

05AB1E, 19 bytes

áΣl}.γl}εgyθ‚}Σl}JJ

Try it online! or as a Test Suite

05AB1E, 19 17 bytes

áΣl}.γl}éεgyθ‚}JJ

Try it online! or as a Test Suite

Explanation

á                  # keep only letters in input
 Σl}               # sort by lower-case
    .γl}           # group by lower-case
        é          # sort by length (stable)
         ε    }    # map each to
          g        # its length
             ‚     # paired with
           yθ      # the last letter
               JJ  # join to string