Some things I found in my own searches. Hoping someone could help me back these theories up.
Handbrake is free, and i think it can handle xavcs.
Also possibly ffmpeg streamclip I think might handle it.
Someone else mentioned “media converter”, which I think I also recently acquired for fre……
For $35 you can get “xavc converter for (Mac)”, which I hear does the job well and is very flexible.
None of these theories have I tested personally, but I lok forward to trying it out soon.
Kzar