I was unhappy w/ my CNM my last pg and I switched at 31 weeks. It was totally fine and I'm SO glad I did it. It did end up costing a bit of money, bc the contract w/ original CNM was a lump sum, and new CNM discounted, but not a ton, so that is something to keep in mind. Also, when I was finding providers, there were many that wouldn't take me bc they were full or they didn't take moms that close to their due date, so if you have someone specific in mind, earlier is certainly better.
I'm in your DDC and I'm still on the fence completely. In fact, I've kind of got my head stuck in the sand. LOL. I hope to pull it out and pick before 20 weeks.
In terms of doulas, I think it really depends on where you live and how popular doulas are. Here, doulas aren't very popular, and it is common to pick into the third tri. I picked my doula around 33 weeks, but I know in some areas, you need to be on that one.