It probably depends a lot on map size, number of civs and gamespeed.
The bigger the map the more cities to take to compete with other empires or to control strategic positions, the more Warmonger points, the more reason to denounce.
More civs = more sources of denouncing.
Slow gamespeed = more rounds of denouncing.
Do you play Giant Earth map (Ynaemp) with all civs on marathon game speed?
I do think some of the warmongering needs to be fixed but in general if you want to go an make lots of surprise wars, raze cities and genocide races then you are going to win the game easily... the best they can do is to annoy the hell out of you for taking such an easy brutal route.
I usually do not start surprise wars. In most games I settle peacefully but when AI neighbours declare war against me I take the opportunity to strike back and correct borders.
Taking over cities by war or peace treaty is about the same (cities change owner) except that one creates warmonger points and the other doesn't. You not always have the opportunity to get cities in a trade and the game also does not run forever so you cannot wait forever for the perfect diplomatic coup which gives you a couple of cities without diplomatic penalty.
Razing cities in Civ 5 / Civ 6 is genocidal because the devs implemented it as genocide and not as a displacement of population. (Compare with Civ 3, ethnic population, etc.) However when I want to build a Suez canal or Panama canal, I don't have a problem to sacrifice (raze) some AI cities for the strategic gain.I think in Civ 4 and Civ 5 razing cities was more important to correct the bad city placement by AI. In Civ 6 you usually don't care (except for the canal cities.)
Everytime the AI denounces me, they give me a Casus Belli, which is a stupid strategy when my military and economy power is a multiple of theirs and we are neighbours.
In real history a strong empire usually became hegemonial power of a region and smaller nations either formed an alliance to destroy the hegemon or came to an arrangement and maybe had a profit from common trade etc.
The denouncing is stupid (or it is badly implemented when they try to exchange diplomatic status with other civs via denouncing. I would expect 2 civs talking in secret about a 3rd civ they don't like instead of both coming to the 3rd civ and denouncing it in public unless they are willing and prepared for war which AI usually is not.)