Hi @jfc, and welcome to the forum.
I’m not aware of any wiki engines built in Elixir / Phoenix (after a brief search, but I’ve also been snooping around these forums for a while and think I would have noticed). The gitgud (GitGud, GitHub clone entirely written in Elixir) has wiki on the to-do list.
MediaWiki is a pretty big codebase so your pathway forward will depend on how you use it and the user base. Search the forums for CMS - you might get inspired by something that isn’t a wiki, but is close enough for what you need (e.g. https://changelog.com/posts/changelog-is-open-source or https://github.com/infinitered/thesis-phoenix).
Converting to/from MediaWiki markup format shouldn’t be too bad, so don’t let that hold you back. Pandoc (https://pandoc.org/) has all kinds of conversion options if you don’t want to code your own. It’s pretty fast too. I use it for quite a few documentation related things - e.g. I can write MS Word docs without using MS Word.
As it happens, John MacFarlane, who wrote pandoc, also wrote a Haskell based wiki - https://github.com/jgm/gitit
