Has anyone attempted to use https://github.com/machakann/vim-sandwich to create a "surrounding" which starts with ":" and ends with "" (blank), to change strings into keywords, for example?

https://github.com/AndrewRadev/switch.vim maybe this is more suited to the task

https://github.com/AndrewRadev/switch.vim#clojure now I feel silly 🙂

Is there a plugin that provides a fn usages command?

vim-replant has find symbol https://github.com/SevereOverfl0w/vim-replant/blob/master/README.adoc , can do something like this http://imgur.com/SvnsLL0 is that what you meant?

clojure-lsp does as well