What I wished I'd known before my books were published:
Not a damned thing. I'm glad I didn't know what a weird and wonderful and stressful trip being published would be. I'm glad I didn't know about awards and nominations before hand. I'm grateful that I had no idea that business relationships could or would twist into something unrecognizable and actively harmful. I'm also grateful that I came to my first several books filled with aspiration and faith and freedom. Not that I was writing whatever I wanted - I recognized the need to comprehend craft and story structure and to honor to contract made with the reader. But because I lived in beginner's mind, I came to writing without any preconceived thoughts or ideas about what it HAD TO BE in order to make a sale or hit a list. When you're out there in the pre-pubbed trenches, you can't imagine, nor should you, the slings and arrows that come with being published. I suspect that when you're newly published, you can't imagine the problems that come with being in demand or with scrambling to make a living from writing. The truism is that we don't get to run away from pain. We only get to pick our pain. Which means every stage of writing life has its issues and its rewards.
This is a profession. It's a job. Like any other job, you'll have good days and bad days and a lot of boring, grindy days in between. That's why it's so vital for the process of writing itself to be the reward. The doing has to be the thing that brings you to the keyboard everyday. If you live only for the results of your writing, you'll have a lot of hard days in before hitting The End. Control what you can - spoiler alert: the only thing you control is you and your writing. Accept that all of life is a learning process and the day you're done with lessons and possible struggle, it will be because you slid into your grave. There's a grace to not knowing everything and a particular sweetness to retaining the capacity to still be surprised.
So no. I don't have regrets about what I didn't know. I'm grateful for what has been and for what might yet come to be. Right now, that's enough.