Upload from iPhone stuck at "Preparing video"
I solved the problem I had - also in this thread:
Video stuck on “Preparing Video” - Apple Community
simply by not uploading from the Photos app, but from the dropbox app.
(Other file storage/sharing apps I don't know about)
So, DON'T use the share option from Photos, it seems to try and compress the file anyway.
in the Dropbox app: just go to Files>Upload. You get straight to the photos/videos.
Also nice to be rid of the hassle of using itunes and all that imho.
/Dim
iPhone XR