Writing advice for a junior PhD student writing a single-author paper
In all my previous papers, I have always had experienced senior authors looking over my writing, telling me what's appropriate and what isn't, how to handle specific things, how to best present results to appease reviewers and editors and, of course, significantly polishing my manuscripts

You want to continue to have all of these things, just perhaps under your own power and at a more complete stage of development (not necessarily when you are almost finished, but when you know clearly what you want to ask). You've hopefully collected some of the advice you've gotten in the past on your work not just as specific advice on that paper but as direction on your writing in general.
However, you still want to have senior advisors look over your writing. Writing a single-author paper doesn't separate you from the need for that guidance. Even tenured professors share their writing with peers (and students) for feedback.
If you are unsure if an approach is appropriate/how to handle something specific, ask an advisor. It's your work, so don't ask &quot;what should I do?&quot;, ask &quot;do you think X or Y would be better?&quot; (it's okay if the answer is Z), but don't be afraid to get advice. There is a lot of feedback you can get without sharing all of your writing - you can get advice without someone needing to read 40 pages to give it.
The polishing stage is probably something you need to take more control of, but once you've polished it as best you can, once you've let it sit for a few days before coming back to editing it with a fresh view yourself a couple times, you'll still likely need someone else's feedback. Few people can write well without it.
If possible, you may be able to split some of the burden to more than one person. Your peers can help as well - and you can trade your own efforts on their projects - and hopefully this can limit the advice your advisor needs to give.