I'm not sure it would. What would extend it is if the phone could just pull from the wall directly & not the battery, so the only charging / discharging the battery gets is from being unused & slowly re-topped back to 80-90%.

If you do your timer idea it seems like either you're going to be hitting it in the 80%+ recharge over & over & over again, which doesn't seem meaningfully different from leaving it plugged in? Or you'll be in the sub-80% quick-charge zone, which will destroy the battery even quicker.