I use Corel videostudio 18 but I am more of a still image person so there may well be better solutions that someone more into video can suggest. The Corel software is very capable but I wouldn't call it user friendly. You can sync manually by moving the audio clip within the timeline but the easiest method is to record the audio with both your T6 and the external device then use the multi-cam editor function and it will automatically sync the two data streams and then you can kill the audio from the T6 when rendering the output because it will only be used as an audio "marker" for syncing the higher quality recording.
Others will likely have different software preferences. I only spent a short time doing research before buying the Corel software because I needed to do some quick editing with video from my XF-400 camcorder.
Since this forum removes most links, rather than post a link do a search for: corel videostudio audio sync youtube and you will find a short youtube video that shows you how to easily auto sync using the multi-camera editor function in the Corel software.
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