Please keep in mind that LE is reduced to just the required components for KODI and hosted in a readonly root filesystem. All additional things (more than a simple script), which are not already available in the base image or as add-on, you want to install should be a package. This can be an add-on package or directly included in a self build image. Many things are documented in the Wiki, at GitHub and also in the code itself.
As for your example with the Python background process, you can set it up manually as you have already done, or create an KODI add-on (script or service) that does the same. Adding new additional Python libraries can be significantly more difficult and depends heavily on how experienced you already are with compiling and packaging Linux packages.
Maybe I'm just misunderstanding you. But your wishes/goals sound very ambitious and require a good understanding of hardware/specialised knowledge in some areas. There is no such thing as a simple all-in-one guide. You should also not ignore the fact that not everything can be realised or is necessary on every hardware platform.
Some starting points: