I have the Mastering Emacs book and read through most of it, which gives me a good idea for some stuff... but I really would like to have good ideas for the right kind of design patterns for common things.
I don't have a good reference (other than the Elisp manual) for high-level design patterns, but in general, Elisp is very flexible and already has what you'd need to implement any patterns you know from elsewhere. Decide on your data structures first. Probably the most important lesson you might not know yet is to use buffers, not strings, to do most text manipulation.
Pick a quality model, even good local ones are fine, you don't need to feed greedy capitalistic machine. Talk to it. Your first step would be to find a way for the model to "peak into" and change actively running Emacs instance - that's typically done through emacsclient.
Next step - choose a harness package - ECA, gptel, agent-shell, etc., with the goal of running the AI from inside Emacs. You can use external tools - pi, claude, copilot-cli, etc. to get there.
And then just ask questions. With utmost curiosity. Like: "What is lexical binding", "why did you write it this way", "how pattern matching is done in Elisp", "how do I make this type of window to always appear on the right", etc.
Best way of learning is by building something practical. In your case, you'd be building your own Emacs. Just the way you imagine it. And that's virtually the only limitation to it - your own imagination.
Protesilaos has a book on this topic. Might be what you are looking for.