can I suggest that just the mixer be moved out of basic computing and into construction 4?
concrete mixer 3 uses maintenance 1 and no computing resources yet is researched in basic computing.
I don't know why gold smelting is a prerequisite for a concrete mixer.
besides the initial concrete production research, mixer 2 is under construction 3.
the only thing necessary for mixer 3 is cp4 -- so I don't see any reason why it can't be moved into the construction 4 research, or perhaps (if it's desirable to have it require lab 4), next to conveyor belts 3.