From your further description, it definitely sounds like something hard to do without significant dedicated staff -- like I could believe it is not realistic to just "run your own mail server" anymore, when mail isn't meant to be your business. For better or worse, you need to use someone where mail is their dedicated business, with the scale to devote significant resources to keeping on top of it. (an "anti-spam guy"!) Which didn't used to be the case in olden days. But, as you say, that doesn't mean it needs to be one of the big three.

Is my conclusion at this point.

But a "bit" (lot) more is required than one interpretation of "just taking spam seriously and having prudent rules in place" might suggest!