I can tell you my experience. I sent a CS9 to S&W for conversion to DAO. I had on mine a single sided safety, which they took off. I also had them replace the guide rod and recoil spring. Altogether with shipping it was 200$ flat and took 3 months back in 2010. If you want to keep the single side safety I believe you can send it back to S&W for de-cock only but then you have the long DA pull you didn't want. Maybe look at some of the threads BMCM has posted about his .45's, he's done a lot with de-cock only I believe, him and a few others.
Try post 45 in this thread
Compact 3rd generation pistols
I haven't seen that member around here for a while but he did have some damn good 3rd gens that he sent back to the factory for de-cock only and the fitting of a Ed Brown oversize single sided safety but I can't find that thread right now.