You're not alone. I find it crazy that you're supposed to write a callback handler, and then register the handler inside a for loop over each server. This feels more like programming inside a framework, than writing a config file.
For complex things, it's nice to be able to do this. But the benefit of having a built-in client is that it works out of the box. This looks like it requires more configuration than many of the existing (n)vim lsp clients, even for the most basic thing: setting up the keybindings.