I think the most important mind shift is from "let's make this extendable by plugins/scriptable so we can modify it while it's live" to "if requirements change, let's just change the source code and redeploy".

I also disagree with the SOLID principles. KISS is more important than adding extra code and sacrificing performance to allow extension without touching the original source files. Unless you goal is explicitly that.

You're trying to write the simplest, most straight-forward encoding of the solution. If you can avoid duplication and make the code read well, you're golden.