Syncthing may not have its own Web-based file browser but a regular Web server (like Apache or ngninx) can show a list of files in a directory without much configuration. Just point it at a shared folder. You could configure a fancier file browser like Filestash, File Browser Quantum, or even Nextcloud if you feel it’s worthwhile.

Likewise, Syncthing may not have its own concept of a “main” hoster, but it doesn’t need to: you can decide what “main” means to you. Perhaps the one you designate “main” has different ignore patterns, or a longer retention policy.

“Keeping some files remote” can be simply making sure your ignore patterns are set how you want them, if that works for you.

MODE=0022 sounds like user perms are different from group and other.

0022 in octal perms corresponds to u=rwx, g=rx, o=rx.

I don’t know if udev “MODE” is the relevant thing here but you could try 0002 so the user part and group part are the same.

Do you mean you want to put the resulting image files into a particular folder on your computer?

I would have thought anything to do with folders and stuff is up to your scanning software, not the scanning device.