If you want, here is my framework when I am full of frustration but seek answers:
You fool, humans are flexible enough to get used to slow experiences. Even if the average user needs to have discord, slack, 100 chrome tabs, word and any other electron app opened simultaneously, he will just go through his work. He may not be happy with it but still continue without changing his habits.
But to be honest, I goddamn hope you are right!
You can totally do that in nextcloud. All your file structure is keeped in the directory of nextcloud. The database only keep metadatas about what is shared and such things.
One soft that strip the file structure and store it only as metadata in a database is Seafile.
For your usecase, you can drop your files in your nextcould directory at the right place and invoke the command occ files:scan. It doesn't watch for file changes, but you could certainly setup a Cron or a script to invoke it remotely.
The Fxtech Pro1X experience was enough to make me run away from all new "Linux phone" projects.
Looks like somebody made a generality out of the Shamir Secret Sharing PayPal implem (read it thought)
In France, I have 3 different banking apps that works totally fine with eos. I don't know if it would apply to your case, but it might be worth the shot