I've worked with O'Reilly and a few other publishers on a proposal that was slightly different to what they usually publish. I went through the same submission process, working with an acquisitions editor, similar to how it happened in this case. In my case, O'Reilly rejected the proposal after a month and half. The feedback in my case was that the book competes with another project, which project got priority. Sure enough, I took the proposal without any changes to another publisher, worked with the acquisitions editor there, had the proposal accepted and we went ahead with the writing process with a development editor.
In the end, working with the other publisher did not work out for me. They did not publish the type of book I was writing, and the feedback from the editors kept nudging me to "shape" the book into the type of book this publisher knew would sell, and would sell well. Unfortunately, it didn't feel like the type of book I was excited to write.
A year and some time later, I'm still writing the book -The Software Engineer's Guidebook[1] - and if all goes according to plan, I'll publish it early next year.
What I've learned with working with a book publisher, is just how involved the process is. Now, instead of taking a $5,000 royalty upon completion of the manuscript, I'm budgeting to spend around $5,000 out of pocket on development/copy editing, cover design and production editing. The things that the publisher would have otherwise paid for. It's a bit simpler to hire professionals these days for these areas thanks to sites like Reedsy[2], which I do recommend for fellow authors self-publishing in finding support on various publishing tasks.