All of the above shortcomings and required workarounds really stress the need for a purpose built panel mounted controller. The market is new, but rapidly growing, and makes economical sense to jump in.
Some desirable traits would be:
A flat profile to be recessed behind the panel frame.
totally water proof.
MC 4 receptacle or wire pigtail for "panel in".
Robust terminals for "controller out" sized for big wire for long runs.
Strain relief for controller out conductors.
Clearly labeled selector for all battery
Earth bonding lug to connect to panel frame.
Multi-lingual instructions with pictures.
Non of these items would add much cost to the manufacturer, and would provide marketing advantages for sales.
Renogy probably contracts out for their controllers, but could specify exactly what they want, and have them in a few months.