Is exp based on # of units sold? If so, it seems like maxing out sell time perks for all units until you reach max level (if there is a max level) is the best way to go.
{Suggeston} Have quest upgrades give bonuses to other quest upgrades in that guild, so that we have a reason to level the early quests that don't do any good endgame.
when I switch tabs, only the mouse production stops working, the trucks keep on going though, so I'll switch back to the game and my cargo will be empty.
Int also allows you to find enemies faster. Indirectly leads to faster xp gain.