For me SM3 stumbled because of trying to cram too much into it.
And yet - as you point out - Sandman was a lame-ass villain, and Harry had become a one-note revenge machine. Who then gets partial amnesia to make him a good guy again - how contrived is that? I never much liked the Harry character, but by resetting him, they made him even more annoying.
Makes me wonder if maybe the studio saw that Raimi was cooking up a big fat nothing, and desperately demanded Venom's inclusion in order to give the movie some
kind of menace?
scores major points by me for not having Harry around. His and Peter's friendship always felt forced and unconvincing, written to justify Peter's connection to Norman more than anything else.