My rose minimally, less than picture. Is there a "trick" to mixing this thick batter so as to do it quicker, and not "overmix"?

See article 553396 at livestrong.com for various ways to cause and thus avoid banana bread collapse. Inaccurate measuring of baking powder one if the top causes.
As for mixing, quickly and lightly combine the already mixed wets and dries, and pour immediately into prepared pans and bake. Do it more as a light folding, rather than thorough mixing with whisk or electric beaters.