Personally I'd always play very hard pirates and do not consider the current situation to be overwhelming or to be a tower defense game ... on any setting.
Oh I'd probably play them on hard too, in fact I wouldn't be adverse to the top level of difficulty making them even harder than they are now. And sometimes I would play with weak, pathetic pirates for a different game experience, in fact I would vary it a lot. But that's just it isn't it? Pirate difficulty CANT be varied except indirectly through other changes to the setup.
Few pirates leads to a few ultra powerful pirates.
Many pirates leads to a few ultra powerful pirates plus many weak/crippled ones.
No matter where you set it you end up with pirates being a serious threat (except with pirates off of course).
If that was supposed to be the way we adjust difficulty, it simply doesn't work. The devs need to recognize this and give us a difficulty slider that actually changes the overall challenge that pirates present. A slider that globally adjusts their income might be a way to do this without presenting extreme coding problems.