To understand correctly: You have friends that care about moving files of their mobile by sync? Or is their only concern switching to Signal, but the lack of backup besides on the device?

Asking because:

• I have the same solution as you but the android folders - foldersync - Nextcloud variant.
• My motivation comes from privacy / sovereignty.
• My friends lack this motivation so they are happy using the cloud (iCloud or Googles) to "backup" everything.
• Any hassle by not using that needs strong motivation to endure and overcome.

So when their photos are in the cloud and their documents stay happy in mail attachments often there isn't much more to backup besides configuration. Which all those bigphone (Samsung Lenovo Apple etc) suppliers provide.

For the chat backup:

• I got my whole family to switch to Signal (at least with me and for group chats, we have them for events and topics). The leverage I used was to only use Signal for baby photos of our newborn kids which my sisters and mom just couldn't live without. 😎
• They complain about running out of local storage and not being able to bulk delete media in Signal.
• As a workaround they sort by filesize and delete a dozen big files (eg video).
• Or they leave a group, delete it along with media and join again.
• I was surprised they don't care about the files after consuming it (foto video) and happy delete them.
• After that ... the backup becomes small again.