Hello!
How come the internal storage will only be good for 55 minutes on HD video (30 frames/s)?!
A regular 32GB card should hold 3-4 hours of video. I see the internal available storage at 22GB, so that should give us more than 2 hours!
Why do I get less than an hour?
By my calculation DJI eats 22GB in 55 minutes, that is 400MB per minute. THAT is CRAZY!!! That should be for FullHD at 240frames/s, not 30 frames!
What can I do to use better compression? I am already using the HEVC mode, doesn't do much ..
Changing from HEVC to h264 gives me exactly the same 55 minutes 41 seconds! So not even 1 second better.

Not only time and fps make sense, bitrate make huge impact on file size. As an example you can download movie in 1.5GB file or 10GB+ file.
In my files with 2,7k/50@ 16:9 , bitrate ~ 67Mbit/s and 2 minutes video has almost 1GB size
67Mbit/s /8 * 120s ~= 1000MB  => no magic

in other video with the same settings bitrate are 80Mbit/s, so bitrate is not a constant so video with same length may have different size

2
cam didn't recalculate remaining time after change settings of codec even after turnoff (this is another bug in firmware) if you change codec and make a 1-2sec video, after rec stop you will see that remain time changed. in my test i received about +2min to length with HEVC on empty internal storage

Don't forget that low res files are also created, and no, that can't be turned off.