I'm still confused why you'd want to purposely cancel a cast so you can cast something else. But if you want a single button press to "stop" a cast, try pressing escape (or whatever button it is for console players). This forcefully stops your current cast as long it's within the range where it can be interrupted.
As for losing procs as a bard, there are actually intervals for river of blood between each procs. You could easily finish your current cast to use whatever proc you have after that aside from Straight Shot.
Even if you consider the chances of losing procs:
River of Blood has a 2x 10% chance to proc, assuming 20% crit rate
Straighter shot 20% chance to proc
Odds are much higher that you don't get a proc in comparison to wasting a proc. Where in most casts you may interrupt your cast for a proc and don't get a proc after your next cast; You simply lose GCD time like this.
It's a bit more complicated for Machinist where reaction speed's a much bigger factor