Is it reasonable for an author to withdraw an accepted paper because they have since done better work?
No, this is not reasonable. It's up to you to determine that your paper meets an acceptable standard before you submit it, and stand by that determination unless you find a critical error in the work. Maybe it's not as good as what you're working on now, or maybe you're just overthinking things out of perfectionism, but it doesn't matter. Not every paper is a breakthrough, and that's alright.

Practically speaking, one less-than-exciting paper isn't going to give you a reputation as a salami slicer, and even if it did, this is much less bad (think several orders of magnitude less bad) than having a reputation for wasting editors' and referees' time because you won't stand behind your own work.

I'm not sure about the legal fine points, but you might not even be able to retract the paper at this stage without the editor's agreement (and why would the editor agree if there's nothing wrong with the paper?).