Oops, I missed that: “Though I’d like to invoke it by holding W instead of pressing it then pressing B to get back to Brush Tool”, sorry. Unfortunately, as far as I know, this is not possible in Krita, this might be inconvenient if you switch to Krita from another software, but you will have to get used to it, at least if you want to enjoy the benefits of Krita 
hehe
But I think a switch to Krita is worth it, even if you always find a tool that is only available with software XY, you will find tools here that are not available with others.
But here you have the chance (NOT the certainty), with a well-founded “feature request”, in which you describe your desire exactly and justify, to get your desired function EVENTUALLY nevertheless.
For this, this idea must be technically feasible, and the idea must find enough advocates, supporters. If all this is true and the developers have the time and can realize it, implement it, then you can eventually be happy about YOUR FEATURE in Krita. But the road named wish list is long, currently there are (I think) over 400 wishes. That means the better your description is, and the more supporters it finds, the faster your wish will reach one of the front places. Just a guarantee that others will wish for your feature as much as you do doesn’t exist, so please don’t fret, that’s life.
Michelist