Hire an editor. Trust me—I’m an editor.
A while ago, I wrote I know how to write. Why would I need an editor?
To add to that post, I’m going to briefly explain why you need an editor for more than just proofreading.
For the same reason that your eyes normalize spelling and grammatical errors, being in the midst of your manuscript for months and years makes all the loose ends invisible to you on the paper or computer screen. After all, in your mind, you know the story forward and backward and how each character lives and interacts with the other characters.
In reality, because you are so submersed, there may be storylines that have dropped off and characters who have disappeared, yet you don’t notice.
There may be a scene where you describe a character using his favorite green pen, while later in the book his girlfriend borrows “his favorite blue pen”.
I often read characters named similarly (Laurie and Lori) or the same (Jamie Smith and Jamie Rogers). This is completely distinguishable in your mind, the writer’s mind. But it can be confusing or distracting for your readers.
In the long run, the amount of work you put into your manuscript is up to you. It makes sense that after years of work, you want a pristine manuscript, the best version of your story, to submit to agencies and publishing houses. Write, rewrite, workshop, use beta-readers, and when all of the manipulations have been finished, then it is time to get an editor.
Should you want it, I will honor your request of only proofreading for spelling and minor grammar mistakes. But I will suggest that you go for a full content edit. This is not to get more money out of you—it is to save money for you. Once I send back your basic corrections and suggestions for a better manuscript, you may notice there are so many more things that need fresh eyes and modification. You will likely want a full-content edit at that point, and then I have to charge you again.
In a full-content edit, I will cover grammar, spelling, punctuation, typographical errors, consistency, language, development, character POV, formatting and citation, suggestions for reworking of clarity and plot, of tone and mood, along with written feedback on writing, story, characters, acknowledgment of strengths and ideas for improvement. My turn-around time for novels is 2-4 weeks, contracted. However, I nearly always return far before deadline.