I don't need 200+ key combinations off my leader. I have about 10 for the things I use often, and if I find myself pulling up commands from Alt+X (mapped to space x) regularly, I add that to a new leader mapping.
Use-package is really convenient to learn for setting up your config.
Other than that, the things I find I can't live without:
* Ivy/Counsul * Company * Flyckeck * Flyspell * Evil
If you take a look at spacemacs's GitHub, dive into their "layers" and pop open the packages file for any given layer for inspiration of other things to add for your desired languages/features. But integrate then yourself given the packages' instructions. You'll be far better off knowing your setup intimately than learning to skim the very large surface area of spacemacs.