This is 100% correct, the GoPro Max (or Fusion) does not support external microphones.
When recording our flights I tend to use a setup along the lines of this.
GoPro Fusion mounted on left wing tiedown (or GoPro Hero 6 mounted on left wing tiedown looking at plane)
GoPro Hero 8 mounted on right wing tiedown looking forward (GoPro Hero 4 with extended battery pack if it's a longer flight)
GoPro Hero 6 mounted just above TailWheel
GoPro Hero 6 mounted in cockpit window (looking out) with microphone adapter capturing cockpit audio.
GoPro Hero 8 in shirt pocket for handheld video or photo's.
The reason being that I find the 360 view just works better when mounted outside the airplane. When mounted inside the airplane distortions are evident whether it be due to stitching issues from being so close to the suction mount, the lighting inside vs outside leaving a visible division line in the picture etc.
For background audio noise (engine noise etc) I have a small audio recorder that is much more suitable for recording as it has adjustable gain levels that can be set before recording. That way there's no overdriving of the audio leading to pops or audio loss. Plus it's handy to place anywhere in the cockpit or cargo compartment.
