Can we assume you are talking about using your mavic as a camera and not flying it during the ceremony, you did not make that clear?
You don't need to get a Rode or Sennheiser, there are a few quite good mics for just about $30 take a look at youtube for some user testing videos to see the best one, but time is of the essence here for you. The best thing to do is record from start to finish if you are after the vows section, so that you have all the audio, then you can cut between scenes if you walk around with your mavic camera filming. Hold still for sections then move and hold still again.
Make a noise like a clap at the start so you know where to sync your sound as the video starts when editing. Then when you come to edit, the sound can stay and you just cut in and out of the video, leaving out the moving about to a new view point parts of the video, not the slow moving you may wish to keep in. The sections you cut out, you can fill back in with still shots, like close ups of the ring exchange, which you can take close after the ceremony. that way you have the video and sound synced plus you won't have any blank spots because you will fill those with still shots taken by others.