I do many 5.1 recordings for doc. Don't forget that the post can lower the rear channels.
A good practice is to focus on the front stereo (or even mono if there are dialogues) and to think that 5.1 recording is an option.
Double MS is very good with that, you can boom it with a MK41 Schoeps for front and post will decide, and choose to keep stereo or 51. You may also add your wireless on another track if needed.
Another option is to work in mono and to record separately atmos in 5.1.
IRT cross will give you a nice space sensation, but DPA5100 will work also.
Vinc.