Fennel is indeed nice and I rewrote my config in it too, but looked back ~2 years later and rewrote it again in Lua. I think Fennel for configuration is not justified and just adds complexity. Also the tools are not there: two existing language servers[1][2] can't compete with Sumneko's Lua language server[3] and they are fennel-exclusive and clueless about Lua code. I still like Fennel for writing more complicated code (my plugins: [4][5]) because of neat features like pattern matching and structural decomposition, both are surprisingly robust.
[1]: https://git.sr.ht/~xerool/fennel-ls/
[2]: https://github.com/rydesun/fennel-language-server
[3]: https://github.com/LuaLS/lua-language-server