Interesting idea! Your suggested scheme would probably help with ergonomics and would be more familiar to players who come from Magicka. Being unable to move while mixing is a non-issue, you can't do it effectively with the current controls either. That being said, there is the drawback of relegating the secondary fire input to a less intuitive binding, which might make no difference in the DD57 build but could (and likely will) prove to be problematic in the future.
What if I moved dash to space then used shift as the alchemy key? You'd hold shift, use QWEASD to mix your stuff, then let go of shift to return to normal controls. The alchemy key could also be a toggle, similar to the old mixing menu, but I fear this would muddy up the user experience by adding more buttons in the way of combat. It could result in some frustrating experiences where you forget to turn off the alchemy mode and try to fight an enemy, only for you to not move and for your mixture to get all messed up (picrel).
Additionally, the alchemy key could clear your mixture for you, removing the need for the "clear mixture" binding. This would free up the tab key for other stuff.