Hi Turniphead,
Congrats on your offer, and I wish you the best of luck whatever you decide. My first book is being published by big five publishers in a few territories next year, and I thought it might be useful for you to hear what that process has been like for me so far, in terms of timelines, editing and marketing etc.
I got my agent last year - think it was late August/September 2016. After I accepted the offer of rep I waited a few weeks, then got editorial comments from my agent. I spent about a month to six weeks doing that work, then went out on sub. I was very lucky and got offers quickly. Once I accepted an offer, the following happened:
I signed a contract (around beginning November 2016) and was paid a generous advance. Waited about two weeks and got a 17 page letter with structural editorial comments, put together by 3 editors across two publishers with more than fifty years publishing experience between them. I worked on that for 8 weeks through December and January. Sent it back to my publishers at the beginning of Feb. Three weeks later I got a fully marked up manuscript back for the first copy edit, plus a couple more pages of structural edits. I worked on that for about a month, then sent it back. Three/four weeks later I got the manuscript back again for another full copyedit. I had about three weeks to get that done and sent it back in around the beginning of May. My publisher has been working with a cover designer for months now on the cover - I have seen early concepts, and more advanced concepts and have been fully and generously asked for my input at each stage. My publisher is in the process of doing a proof run which will go to influencers - bloggers, book-buyers at major retailers, etc etc. I have completed an author questionnaire for marketing purposes and I have a publicist assigned to me whom I can call at any time. She has a whole list of stuff worked out for me that we are not getting into right now because my lovely publisher is giving me time and space to work on Book 2. I will get Bk 1 pages for proofing I think in about three weeks (I have a detailed schedule but am too lazy to look it up now). All of this is for a book (Bk 1) that comes out next March!!
Yes, that's March 2018. And honestly, every inch of the way has been a joy, a pleasure, and I would do four times the work all over again. I get to work with editors I deeply respect and admire, and I could not be more supported. Believe me, I get the impatience, the wanting to get there right now. And it is perfectly possible that your small publisher is exactly the right fit for you, so I am absolutely not saying not to accept the offer - only you can make that decision. I just wanted to take a moment to remind you that when you get out the other end, best case scenario, you have a chance of building a writing career. It's a small chance, even with a big 5 publisher at your back, because there are so many quality books coming out all the time. Ask yourself what you want, and if it's a long term career, ask yourself who you want to work with on the road to get there.
Best of luck,
Maggie.