Here’s how Discourse would work for your use case:
Set to invite only.
Solved via Configure an S3 compatible object storage provider for uploads
Use Discourse on a mobile device and see if it meets your standards.
They are adequate out of the box.
Groups and permissions per category.
It is basic for Discourse wiki topics, but basic works for many folks.
$5/month plus cloud storage (itself possibly pennies a month)
I don’t pitch Discourse as a wiki on it’s own, more like having a collaborative space with wiki-like editing, among the other communication features Discourse uses to keep folks in touch.
Ganbatte! 