A few years ago, I converted my old duty gun (2000 P2226) from DAO to DA/SA. At the time, the trigger felt fine in double action, but felt “ratchety” in single action. When empty, and with a cocked hammer, I squeeze the triggger and just before it hits that reset point, it starts feeling ratchety. Not smooth at all. At the time, I looked and looked but eventually put the project on the back burner. Today, I looked at it again with a fresh set of eyes and I’m still lost.

Can anyone help?

Did you replace the sear when you replaced the DAO hammer? Hammers and sears wear in together over time. It could be that the sear's wear pattern from the DAO hammer is rough against the new hammer. I'd try a new sear, or at least inspect it for burrs or rough spots.

I had a similar issue about 8 yrs ago. I disassembled mine and polished the internal parts and it cleaned up the trigger squeeze and reset.