I suggest that your autocast selection is saved whenever switching weapons.
Anytime you change your offhand or mainhand weapon, your autocast spell is deselected. For example, if you have a staff of eternal hellfire and you equip a tome in the offhand, even that will deselect your autocast spell.
It would be like osrs and would be a huge help when fighting bosses/npcs requiring frequent gear changes if this change was eventually implemented.