Here is a partial answer to many of your questions...We don't know why God does many things, we also sometimes why he partially does things, but not the whole. We have to trust that he did it for good. Now, you as an athiest and God hater, will say the opposite, but since you are arguing from the confines of Christianity, you cannot argue certain things, like God's intention (from a place of lack of understanding).
"Those babies." well, in terms of the standpoint of the Bible, those babies had mounds of generational sin. Many of them would have had generational demons and curses. Now, since the babies were not at the point of knowledge, they are with God.
"It's a common misconception." YOU, an athiest telling Christians what is a misconception. l.o.l
Yes, Noah did warn the people for many years of the coming flood, and until the flood was drowning the people, they scoffed at him. He was an outcast.
He killed the firstborn males just as Pharaoh killed the Israelites babies at a certain age. God was directly punishing Pharaoh for his actions..."Killing a bunch of little kids" is not seeing this chapter through the eyes of what is eternal. Now, the angel of death killed those kids, however, it was their parent's fault. They did not obey, and paint the blood of a lamb over their doorways (symbolization).
Obviously with a poor exegesis.
"That's no excuse. That's like a domestic abuser saying "think of all the times I could've broken your jaw but didn't. Doesn't that make me such a nice guy?"
You are assuming the very thing you are trying to prove. God would be the domestic abuser, except that is what you are trying to prove.
Now, the many times when God spared a filthy nation from His wrath were ones where God's love is displayed (even though there are many other ways that his love is displayed). For example, in Sodom, God saved the righteous from hellfire. The filthy, pedophiles, molesters, and other sexually immoral were destroyed, and rightly so.
Read that passage: https://www.biblestudytools.com/bible-stories/sodom-and-gomorrah.html