I am still working on the cover letter where I am explaning the details about our relationship. Then, I will add an executive summary page. I might have close to 15 pages plus the photos that backup the details mentioned in the letter.
I do not know basics of your relationship or what you are including in these 15 pages.
But unless there is something very unusual or special or problematic about your relationship and application, that is WAY TOO MUCH. I'm not kidding. There are forms. You do not need a 15 page cover letter. Only a report of several hundred pages should have an exec summary as long as a page (or alternatively think of cover letter as your exec summary if you like).
Let facts and docs speak for themselves. And I repeat for anyone else reading: if your app is simple and straightforward, DON'T complicate it by deciding you need to do all the things below. If you don't think you need relationship support letters, you probably don't - things I've put are just some examples.
I don't think you need a long explanation of where you met and every detail of your relationship since meeting. "We met at work, started dating, dated seriously for [time], moved in together [date], got married [time period] after meeting." That's a basic narrative - adjust as necessary. The rest: specific dates and evidence/docs (if and only as needed/requested).
Here's how I'd approach a cover letter - which should be a total of ONE page, and if needed, a second page that lists the documents attached to the cover letter. If there are docs that themselves need explanation, SHORT (one-two para) notes attached to each of them.
1) Intro/Exec summary: one paragraph. A short paragraph.
2) Basic story: two paras. This should be short. Include here a short explanation for any gaps in documentation (eg we cannot provide a lease showing we resided together because [reason]; we do not have joint bank accounts because [not allowed in our country or whatever]).
3) Description overview of docs/appendices attached (eg "As evidence of the time together, I have attached the following documents and evidence for your information: travel tickets and receipts; letters of support from friends and family; correspondence and other between us; wedding ceremony documents (apart from photos which are attached separately). [This could be bullet-pointed and leave out the list in my 4]
4) List of documents and attachments (grouped, numbered.)
I would leave it up to ircc think whatever they want.
They will think there is something very wrong if you send them a 15 page document filled words. To an immigration officer examining your file, that 15 pages is Dostoyevsky. Don't Dostoyevsky them.