Are you set on using windows IIS for this? IIS is a ridiculous pile of absolutely ancient stuff that’s a pain to use, and I wonder if the issues on the mobile apps are from IIS not supporting WebDAV properly.
There are other great WebDAV servers out there, like https://sftpgo.com/ which does support Windows.
I would also recommend getting an SSL cert using letsencrypt if you have your own domain name, makes things much easier. Or if this is local only traffic (or over a VPN), just run HTTP if that’s easier.
MongoDB does have that annoying quirk where it creates several huge files even with a small amount of data in the DB itself. But at least it can be upgraded.
SQLite is really the only one I’ve used that doesn’t bother me in some way.