You may want to pass through the audio so it's not reencoded. The top answer on this stack overflow article has an example. https://stackoverflow.com/questions/9913032/how-can-i-extract-audio-from-video-with-ffmpeg
And a copy/paste:
ffmpeg -i input-video.avi -vn -acodec copy output-audio.aac
Relevant example, with afraid.org you can do it with a wget or curl command.
I feel like these are just growing pains. I mean I'm sure people will argue about to defederate or not for as long as the fediverse exists, but I imagine it'll become background noise as people get accustomed to what the fediverse is and isn't.
The fact that this level of choice exists, both for operaters and for users is a big part what makes these places stand apart from reddit and it's ilk.
Not an answer to your question, but rather an observation. Best to always assume you're being tracked and behave accordingly.
It sounds like your ask is for a recommendation to back up personal files from disparate locations to a server.
I use a self hosted Nextcloud instance for this purpose.