Your question is not a good fit for this site for "enthusiast programmers", but you raise an important issue that I'd rather try to address than shut down.
What you are after is called a SharePoint Governance Plan.
Nobody can tell your company how your company should be using SharePoint. You will need to work that out amongst yourselves. If you have been asked to come up with a document, you've really drawn the short straw. Because you won't be able to write that document on your own. But you can look at SharePoint governance guidance and sample governance plans and work out from there what decisions your company needs to make, what your company wants to achieve and where your company wants to go.
The Governance Plan helps to document where you are now, where you want to be in the near future, and maybe a few tactical steps to map out how to get there.
The samples and articles you can discover when you google SharePoint Governance Plan sample will only be a starting point. The hard bit comes after you realize that your company needs to make a few decisions, which are above your pay grade, but you may not know who should make them. You will need to collect representatives of the business in a Governance Committee, and they will contribute to the SharePoint road map and the governance plan.
If you work in IT, chances are that you will find it very hard to get the commitment from the business to contribute time and effort to this whole Governance spiel. They just want SharePoint to work and expect the IT department to magically mind-read and automatically apply whatever they need without spending time to detail the requirements.
A SharePoint Governance plan is a little bit like a business plan. It includes vision, goals, strategy and tactics and it cannot be written by one person's efforts alone. It has to be a team exercise.
Try this article for a start: https://sharegate.com/blog/real-world-sharepoint-governance-plan
Edit: If the document is supposed to shed light on difficult decisions, you can present different approaches for each of the scenarios along with pros and cons for the approach.
For example:
Challenge: Developing custom solutions in SharePoint
Option 1: hire external developers
Pros: no need to recruit and/or
train in-house staff
Cons: more expensive, also hard to tell if externals have the knowledge to do the job
Option 2: employ developers
Pros: working full time in-house, easy to manage, cheaper
Cons: hard to recruit, difficult to judge applicant skills in advance
Option 3: do nothing
Pros: don't have to make a decision, phewww!!
Cons: Nothing gets done, arghh!
Again, the point here is that it is not you who has to make the decision. You present options and the business representatives evaluate the options and make a decision, which will then be documented in the next version of the governance plan. The governance plan is not set in stone. It changes all the time. It is a living document about how the business wants to use SharePoint.
Things change. That's a given.
The Governance Plan tries to define how things should be handled with the current information.
The Governance Committee meets regularly to review the Governance Plan and they can change the plan.
The whole idea here is that the Plan comes from what the business wants, not what a poor single soul in IT has been tasked to dream up.
