Is there a way I can keep using my defined variable

https://www.gnu.org/software/emacs/manual/html_node/elisp/Backquote.html

As a side note, it would be better to set your variable like so:

(defvar my/mu4e-dir
  (cond ((my/laptop-p) "path/to/it")
      ;;etc

The way you’re doing it in your example seems inverted.

is there any guide you can point me to?

google and the search bar on this subreddit both work great

What have you tried so far?

Seems over-complicated. Most of the “modules” amount to:

;;install package
;;require package
;;provide named feature for module

How is that any better than a single file with some use-package declarations? Especially considering you’re using a literate Org style. The blocks could be toggled with the “tangle” header arg. The Papa Roach Approach (“Cut my lisp; init pieces…this is my last resort!!!”) is an anti-pattern. It makes Emacs load slower and is harder to reason about. If the aim is to grow a community of users, what does this config offer over the myriad other options?

Cut my lisp init pieces! 
This is my last resort!
Suffocation. No layers.
Don't give a fuck if my code signals errors!

https://www.reddit.com/r/emacs/comments/17610on/seaman_emacs_my_first_published_config/k4k0npq?context=3

What type of errors specifically are you running into that break your whole config?

https://github.com/magit/magit/discussions/4997#discussioncomment-7050963

What’s needed is a reproduction case. There are people claiming the bug affects them with both package.el and Straight. I’ve not encountered the bug myself, and frankly can’t be bothered to dig into it if the people who are seeing it can’t provide a repro.

You didn’t mention which Emacs Version, package manager, etc.