What to do with a very long research paper? Some background first.
I recently graduated (a couple years ago) with a Master's in applied mathematics. During grad school I began working on a paper, which I continued to work on in my free time post-graduation. A complete rough draft of the paper is nearly done and is ~115 pages in length. The work contained in the paper is completely original and solves an open problem (the problem is open but not anything necessarily revolutionary).
It was of my opinion that the paper contained publishable material.  To verify, I emailed a professor at my alma mater with a copy of the current draft. The professor did reply stating that the work was publishable and even suggested some Q1 and Q2 journals that might accept this type of work. While this was useful feedback, I received the reply in just a few days so I doubt the professor in question had the ability to read my paper in depth.
The problem:
I have published a couple papers before and thus have some experience in the world of academic publishing.  That said, the scale and complexity of this paper is something I have never dealt with before and so I am not comfortable with proceeding to publish it without help/guidance, i.e. on my own. In particular, I suspect I am going to have to divide the work into small portions and publish a few separate papers but don't know how to do this. Also, the paper is very dense and I am concerned that its &quot;readability&quot; is not exactly optimal. Given that I do not have a ton of experience and am essentially working in a vacuum, I also really desire to get feedback on the quality of my proofs, which I suspect are not as concise as they should be. My situation seems a little unusual and I suspect that the feedback I am looking for would typically be provided by an adviser in a PhD program (not claiming the paper is worthy of a PhD).

Given that I do not have an adviser that can provide detailed feedback, what should I do?

I thought about reaching out to professors/experts with relevant backgrounds and proposing to add them as coauthor in exchange for helping navigate the process of publishing my paper.  Is this an absurd proposition?  I wonder how such a request may be viewed. Are there any other routes one might suggest I consider?
Most journals will make any page limits public. Some will take long papers, but that assumes some significance to the result. The best way to find out is to make a submission to one that doesn't clearly state a shorter limit.
Navigating the publishing process isn't co-authorship and shouldn't be offered.
It might be necessary, however, to split the paper into two or more papers, assuming that there are intermediate results worth publishing independently. In applied math, it might be that some intermediate results have their own application. That might be a way to look at it. But the sum of several smaller papers can be expected to total more than the original.
If you make a submission, the editor and reviewers will comment on the writing and if it is &quot;too dense&quot; will suggest edits, though maybe not in specific terms. But you can decrease the density by increasing the length (more explanation) and this, again, leads to the possible necessity of splitting the paper.