For anyone experiencing this problem, the publisher of the AUR package of megasync solved the issue. megasync no longer requires ffmpeg3.4.
So to make a full update of your system, update only the megasync package using your preferred AUR helper, this will remove the dependency on ffmpeg3.4 so you can now remove it without unistalling megasync.
yay -Sy megasync
sudo pacman -Rsn ffmpeg3.4
sudo pacman -Syu