Considering you can get +30 from modifiers that aren't even the agendas, I think the warmonger penalties are fine.
In 6-8 player games I've been able to consistently ally with at least two civs every time. More often than not those civs are enemies with each other, so I take a -8 hit with them for being allied with the other and they still never care when I DoW and conquer other civs.
Delegations, Open borders, Trade deals, Trade Routes, can get you just under +20. Matching Governments and/or a religion is +5-10.
Declare friend as soon as you can, that's another big modifier.
With my experience, I can't help but feel the only folks who are having harsh repercussions due to war are people that are ignoring diplomacy.
Of course, if you care about the diplo-hits at all, you'll have to accept that surprise wars are pretty much out of the question, and then eventually, you'll rely exclusively on casus belli. Though that's not exactly necessary - it helps.