I always build up my army as I go and upgrade the units with cash, and then maybe build a few units to expand my army or replace losses. One thing this means is that my cities can keep building things while I'm fighting.
I think this is a part of the single-most-important-element of Civilization 6, that being...
...the key to success is to declare the one war you intend to fight in the ancient era and finish it before the medieval era. From that point on, your military is to be kept strong enough and positioned well enough that no one will declare war on you.
From my experience, you will win every single-player game if you can pull off this feat, and with very few headaches along the way. There are several stipulations/considerations. First, it obviously does not apply to domination wins. Second, it is extremely hard to pull off at deity level.Third, you may only discover which war is necessary by the classical era so you'd be a bit delayed in setting up your victory. Fourth, the placement of civilizations may mean that you'll want to have two wars instead of one. This is because of the reason why you're declaring the war in the first place. And that reason is...
Having adequate territory gives you the space to build the cities which give you the yields that you need to win the game.
That's pretty much the whole game in a nutshell. You need enough cities to get your science, culture, production, gold, tourism and/or faith yields to an adequate degree which will allow you to perform the tasks that are needed to achieve a victory condition. Usually, someone else claims part of the territory that is needed to fulfill this agenda, and that is the person that you declare war on. Someone else DoWing you, or being a major nuisance, is not a valid reason for declaring war. There is no emotion; no retribution, that has a purpose, the only purpose is to claim the territory that is necessary to get your yields to a degree that a victory condition can be reached. Granted, there is some role-playing involved as this is a game, there is satisfaction that is felt when you trounce the person who's been a nuisance in this game or previous games, but for optimum play they are a distant second in priority.
Playing the game from this perspective, you'll want the declaration of war to take place in the ancient era because there is no warmonger penalty if this is the case. You will get some warmonger penalty because conquering cities does contribute, and this is considerably more of a penalty if you burn cities down because the AI made a bone-headed placement or if you need to eradicate a civilization.
One of the most notable considerations when you play this way is that early unique units are more helpful than later ones. The cossack may provide better advantages than the war cart, but since the purpose of the war cart is acquisition whereas the purpose of the cossack is either deterring war or concluding it earlier, the war cart is a much more valuable asset.
I've also noticed that War Weariness is not necessarily evenly distributed across your cities. Just this past weekend, I invaded a neighbor and one of my home cities gained -6 war weariness rather quickly, but everybody else was fine.
I've seen this happen too, but often the "office of luxury amenity distribution" will correct this on the subsequent turn.
