Taking a page out of Rocket's (and Slack's) book, I went ahead and created keyboard shortcuts in System Preferences for emoji I use frequently that follow the format ":emojidescription" (e.g. :catscream). The description isn't necessarily the emoji's official name, just what makes sense in my head. They sync via iCloud so they work across all my devices and they follow the convention I'm used to using in Slack which makes adding emoji consistent pretty much anywhere I'm at.
I'd recommend trying a free app called Emojise. I've been using it for a couple years and it's so quick and easy.
I’m a fan of Rocket for Mac. It allows you to insert emoji quickly in a Slack-like fashion. Lives in the menu bar.
Remarkbox