I've had this question asked so many times and truthfully? I don't think you actually know when your book is ready for publishing. Yes the grammar can be down to a tee. Yes, your chapter length could be spot on (through your own eyes) but someone else may disagree with you.
I think, no matter what, your book is never truly ready - and that's okay.
My debut novel: Through Her Eyes - available from 28.08.19, on a variety of platforms: Book Depository, Waterstones, Amazon; paperback & kindle - was drafted at the age of sixteen. I had this idea I wanted to play with and it all sparked from the TV show, Pretty Little Liars. I worked so hard to try and get as many chapters done as possible and it was only until I come back to it last year, taken up creative writing classes, that I finally finished it.
Even then, you have the re-drafting, re-writing, re-reading - attacking it with a red marker until your hands are blue and you can finally change, or correct it all. You still look at it at the end and think somethings missing...or maybe you don't. Who truly knows, because for each and every single one of you, whether you feel like your book is ready for publishing is different for you all.
They say overthinking is the biggest cause of unhappiness.
I can relate to this. Not that i'm unhappy but when I thought my book was finally ready for publication (it had been professionally edited, which was my choice), it had been accepted by a publisher and there wasn't much needed to be done to it BUT I still found myself reading it and wanting to change things. I was overthinking. Overthinking it wasn't good enough, overthinking about how people would take to it, overthinking the storyline.
I could not stop myself.
But YOU can.
Be easy on yourself. If your betas love it, if you love it. Than think, is it ready for publication? Am I happy RIGHT now if somebody picks it up and says it will be released six, twelve, twenty-four months away.
Only you will know.
All my love,