Need help deciding on a Private Wiki product

I'm working for a moderately-sized software business and have been tasked with documenting… everything. There's a lot of info of lots of types, so we decided to go with a Wiki over something like a long Google Document. That said, we tried GitLab and were quickly thrown off my some weird little things (no ability to order the pages, no real page hierarchy, etc)

So here are the points that we're looking for:

• In-house, NOT customer-facing. We have a website hosted and can use that if needed, but are fine using a separate program or website apart from our solution and repo
• Auto-generated documentation not needed
• Flexible and convenient page structuring
• Decent support for images or basic page formatting
• Markdown or WYSIWYG, either is fine
• Page/topic linking or some sort of link/integration with Slack

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Here are the programs I'm choosing between at the moment:

• Nuclino (my leading choice)
• ReadTheDocs (secondary choice)
• Confluence
• MindTouch
• Tiki Wiki
• ProProfs

Has anyone experienced any of these? Would love to hear about pros/cons, either from a referencer's pov or a documenter's pov. Thanks for the help!