You can download the Opus directly with -f 249 / 250 / 251 (~48kbps / ~80kbps / ~128 kbps respectively, but youtube don't always make them all available, where are -f 140 for the ~128kbps AAC (.m4a) is always available, and often the format code 139 (~48kbps) - the lower bitrates being adequate for most speech based content.