I don't see it mentioned but there is a neovim plugin called conjure [1] that is written in fennel. This is a general purpose REPL client that works for Clojure (I use it daily for Clojure and ClojureScript) but also for fennel itself. So you can have a really nice neovim plugin dev experience using this tool. Conjure works with fennel out of the box, but if you want to go deeper you may be interested in another plugin called aniseed [2] which provides interesting facilities for the fennel language and that is used internally by conjure.
Disclaimer: I am not the author of those tools, just a random Clojure dev.
Disclaimer: I am not the author of those tools, just a random Clojure dev.
[1] https://github.com/Olical/conjure
[2] https://github.com/Olical/aniseed