OP: two mic recording via the tascam internals or via a pair of headworn mics will recording the entire sound they hear. To use an analogy, the egg has already been scrambled before it arrived at your mics. Post processing is extremely limited in its ability to unscramble the egg. Generally speaking, the goal is to get as good a recording as possible on the front end with limited post processing. If you're having to crank something in your post processing, that suggests to me there was a problem somewhere in the recording itself. Good recordings are often a matter of a few degrees this way or that way, not a 180 degree change of course settings, in my experience (others feel free to share their experience).
If you recorded the entire band's instruments and vocals separately via a 32 channel multitrack recorder, your options to change the mix would be much better.
Since you can only record what's presented to your recorder, your goal is to position your recorder to record the sound present probably around -12db to leave some headroom and record as cleanly as the circumstances allow.
As far as powering external mics is concerned, you have to research the power requirements of the particular mics you want to use to see if they will run on the O5's internal plug in power (3v or less) or if they will run off a 9v battery box or if they require more power still (true phantom power). Mics that are fed less current than they require to operate either won't operate at all or will distort the recording.
Besides the church audio mics, Jon Stoppable (Naiant audio) also sells a small set of external mics that don't get the same attention on TS that the CA mics get.
It's like everything else, we all learn by asking questions and trying it for ourselves, asking more questions....
