Animation Setup - How the heck to define this

So, for the longest time we have been kinda “winging” this process and it has worked, however the overall quality of the blends and animations has caused serious problems. This is what leads me to here.

How should I setup my animation system? When do you use a blendspace? When do you use the animation states? when do you use a montage?

For this example, I want a different idle state for handling different weapon types. For example (1hand, 2hand, staff, etc), how would I achieve this? Off the top of my head I could create a different idle state for each and make logic that links this to the rest of the stuff, but this could get overly complex and very easily misconfigured. Would this be where I would use a montage?

If you need more details let me know, I just want some guidance on how this might be achieved. Right now I am kinda stuck so any and all help would be appreciated.