The cons are heavily loaded circuits have more voltage drop and inadequate room for expansion.
No, what are you asking? There is nothing wrong with running a double-pole circuit from a breaker with the handles tied. It is two 20 A circuits with a shared neutral, and in my opinion, has slightly less voltage drop for normal loads than when not having a shared neutral.
The problem is that two circuit is obviously not enough.
You just describe at least a 50-70 amp load.
There are two circuits where five are needed.
Run some more cable. Two more runs of 12-3G would be a good start. If the distance is over 50' (wire length), use #10 for the heavier loads.
