1600 with a shutter of 1/250?
You're gonna be up against the max f-stop all the time in most churches. I'd go to 3200 and shoot RAW, clean up the noise in post.
I tried 3200 ISO in low light, and the darks seen on the camera's display would take great heroics to repair. The time needed to repair only the ceremony shots would tolerable but I'd hope to do better. The church ceremony time is the peak and it should look good.
One of the two weddings I assisted at so far had the entire event, ceremony, cake, group shots, dance, all in one place, and it was like a cave lit with candles. There was a huge arched window behind the ceremony platform mixing cloudy natural light off of trees from behind with minimal incandescent light from above. I got the skin tones OK but the darks shot away from the window were blistered with odd tones and the full window archway was in the most important shots, surrounding them in blue trees. I never want to have to do major repair on so many shots again.
I think I can drop from 1/250 sec down to 1/160 sec or 1/125 sec in order to use 1600 in low light. Otherwise I pull out that flash I still need to learn how to use. There is no other way unless flash use is banned.
As far as other settings:
For shooting mode, I did a test and I think Aperture priority is best because with my other settings, Programmed mode sometimes seems to force wide open apertures when it isn't needed.
For Auto-Focus & Exposure (AF-C,S,A and focus point use etc) it seems photographers say "this is what I do" or "do it the way you like". So I assigned U1 and U2 with an AF-S setup and an AF-C setup.
thanks for the suggestion allegretto, it kept me digging.