Hey, welcome to the game, glad to hear you are thinking it's cool.
I'll try to answer some of your questions.
Why aren't giants the strongest? Giant troops struggle with gold efficiency and attack density compared to elite human troops. Many giants can only fit 1 to a square (Gittites are pretty tiny, basically just slightly bigger men, so they can do better here and fit 2 to a square, or even better one Gittite and one Gibbor) whereas humans can fit 3 to a square. Furthermore many giants are going to cost significantly more then a human unit. If you have giants and you're fighting an enemy that has 3-5 heavy hitting human infantry per giant you have, what will happen is they'll meet in battle, the giant will squish one human per turn as meanwhile the humans are landing several strikes per turn against the giant. The giant will kill a couple of humans and then die, and on net the human nation will have lost more troops but less gold.
Of course there's a ton of variables to this equation, how good the giants are, how good the humans are, and if your giants are sacred there are some blesses that work better on sacred giants then on human sacreds. The Hinnom/Ashdod/Gath line of giants are often a bit more elite then other giant nations as well which tends to help them out (of course, the other giant nations have their own benefits)
As for other nations, not every nation wants a big bless, the frank reality is that not every sacred is worth investing points in over getting more income from your scales so you can get more mages and non sacred troops. Many human nations fall into this category where they want only a small bless, sometimes even a bless not to help their troops but to make their mages better, and they focus on either a large monster god to help them expand or income scales to help them build more everything. Although Mictlan is typically a nation that wants to focus on it's sacreds (although that can somewhat depend on the age you're in)
Some human nations can expand perfectly fine. Some do want a expander (typically a big monster) to assist them early on, either because they need it or because getting a large territory early is the best way for them to go forward. Dom6 is much harder to expand in then previous games, so there might be more nations that need a expanders help.