I recommend the CQUI mod (once they’ve fixed it). It really helps to make the diplomatic relationships clearer - not that they make much sense even then.
The big takeaway I got from it is how much the warmonger penalty affects everything. Despite that they’ve already toned it down from the original vanilla release.
In particular, you get a relationship penalty with every civ you’ve met not only when you declare war, but also every single time you conquer a city. This makes it so that as soon as you start on the warpath, there’s pretty much no coming back ever. Everyone else will hate you almost no matter what you do from now on, or have done in the past.
It seems that multiplayer is by far the best way to go with this game… shame they haven’t added good multiplayer support. Check out http://www.sullla.com/Civ6/PBEM4-1.html for an AAR of a really good multiplayer game.