Can't u garrison an archer and bombard first to get the full damage, and then take the archer out and fire with it on the same turn?
units can be on the city tile and not garrisoned. if the unit is garrisoned the unit icon should be floating to the left of the city strength, and the ranged attack screen should list a garrisoned unit: +100% bonus (although it's not really a +100% combat strength - it is just a 2x multiplier)
yes, you lose all bonuses if you fire with the archer first, and additionally next turn it won't start out garrisoned. each turn requires this micromanagement: garrison the archer, fire with city, fire with archer
But 8/10 is equal to 4/5