As I recall, we discussed this in another thread. (here)
https://www.early-retirement.org/forums/f28/dqotd-rmds-before-roth-conversion-rules-118463.html There was some debate, but I think that after we all clarified our positions, the conclusion was this:
1. If your expected QCD plus your tIRA distribution to your taxable account in a year
might exceed your calculated RMD amount for that year, then you should do the QCD first to reap the maximum tax benefit of having the QCD offset some of your RMD.
2. If your expected QCD plus your tIRA distribution to taxable
absolutely will equal your calculated RMD for the year, you could do them in either order.
Edit to add: Personally, I plan to leave my taxable distribution until late in the year just so I have room to increase my QCDs should an unexpected need arise.