I do that a lot too. When doing military i often deliberatly don't take the last city so i can farm xp gor my commander. You have to keep stepping away from the city to allow it to heal then step in, bombard it to zero and step back again.
For naval cities/commanders this is particularly easy as i try to find a clump of cities i can cycle around....reduce one to zero, move to next, reduce it to zero, move to next, reduce it to zero, return to first...rinse and repeat. Then take which i want on the last turn (if i am not just simply farming xp).
With codicies/relics you just need to make sure you orignally get more than you can slot then the excess will only be slotted manually even when you get more space unlike them slotting automatically if you already have space.
All the legacy paths can be gamed quite easily to make sure and era is extended and you fulfil as many as possible. The hardest are usually wonders as you can't control the AI so much (who may complete it before you do) and the 20 resources simply because you are holding your cities back quite a lot by not slotting resources when you have space to do so.
