Whilst I can't compare fulcrum to confluence, I can compare Zim to confluence, and they are chalk-and-cheese. Zim was designed principally as hierarchical notes-on-steroids for personal use. There's no permissions beyond the file level. I have no love for confluence (or jira for that matter); we've used both and fight both. We now use xWiki across the org for shared knowledge -- it has good access controls -- displacing confluence. Data migration was easy. For local information management, we suggest people use the tool they are most comfortable with from {dropbox paper; evernote; zim; devon-think; apple-notes; xWiki}. (We are a Mac & Linux shop).

There is no need for a second infrastructure and I aim for a tighter integration with GSuite Tools.

In Confluence GSuite is an afterthought.

For example when you upload an image in Confluence it saved in Confluence and you can't use it in any other GSuite product. When you upload it in fulcrum you automatically upload it to your Drive and can use it everywhere else.