That’s where the artistry comes in. I haven’t made an advanced locomotion system like ALS, so I don’t know the exact formula, but I would say to keep things as simple as possible and not get overly complex (I actually think the ALS system is a little too complex, but that’s just from watching the video).
For modularity, you could build one base state machine (i.e. the logic), then switch the animation clips for each weapon (i.e. the content) through blueprints. This way, you could have as many animations as you want without having to manually add them to the animation graph. This is like switching skins on a character, where you change just the model and not the logic.
This thread has a similar problem: Can you define poses/state-machines, porgramatically, inside the animation-BP (or elsewhere at all)?, so you can read that for more help.