How can I seek help in preparing a very long research article for publication?

I recently graduated (a couple of years ago) with a master’s degree in applied mathematics. During graduate school I began working on a paper, which I continued to work on in my free time post-graduation. A complete working copy of the paper is done and I have posted it on the arXiv here. 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 (current as of approximately six months ago). 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 of 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 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 I don’t know how best to do this and do not want to sink a lot of additional time into this without any direction. Also, the paper is very dense and I am concerned that its readability is not 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 Ph.D. program (not claiming the paper is worthy of a Ph.D.).

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

I have tried reaching out to researchers/experts with relevant backgrounds and offering authorship in exchange for the help I need. Since what I’m seeking entails a significant amount of work, this proposition seemed reasonable; however, my efforts have not lead to much fruit.

As professional mathematicians, how are these types of requests viewed and how might I go about reaching out for help? I do not have funding and so I considered the offering of authorship as a a reasonable incentive to get the help I need. Is this practice frowned upon and is there anything else I can do to increase my chances of getting a researchers attention?