So what I'd like to do is basically use a second mac keyboard and use it exclusively as a macro keypad.
So for example, I'd press the 'a' button on the secondary keyboard, and it'd trigger some combination like ctrl + shift + f without it actually printing the 'a' key.
P.S. I understand that Karabiner Elements can provide this kind of functionality, however there's a really bad bug for M1 macs which prevents sleep functionality, so I'm looking for alternate ways of going about this.
BetterTouchTool assigns actions specific to a particular keyboard by default. You can easily assign actions through BBT, or BBT's action can in turn be to call a KM Macro.
Amazingly doing a search of this forum for “USB” turns up a wealth a threads that touch upon this question (and, of course, many that don’t). I’ve just spent 20 minutes looking at the search results and there are many that are relevant to your question.
As a taster, look at this one:
Investing a bit of time to look through what’s in this forum will go some way to helping you out. Good luck.