I found an excellent video on YouTube by AstroBlender that goes through his image train and backfocus calculation on a Celestron EdgeHD 9.25. His setup is very close to mine and he suggested the ASI174mm for the guide scope to better match the prism in the a Celestron OAG. I just wish it wasn't so expensive.

My ASI120MM mini just can't see much with the OAG and is not practical for guiding.