Agree overall, but I disagree about the mom and the monsters' variety.
The point of the story is to unveil slowly the mystery of the mom, the birth of Atreus, and what Kratos did with this new family overall. So it's normal to not have all the info at first. The end game twist is literally the explanation of the mom's origins, the fact that we learned about her slowly during the whole game was greatly done IMO.
For the enemies, you're exaggerating. Sure, the most powerful ennemies are not that different. But you have overall a large enough number of ennemies. When you do the Muspelheim trials where you have to avoid all hits, you should realize how many different ennemies with different attacks there are, with different strategies for each. It's surely more than most games, no complaints from me here.
About the mother, it's like this. While you're playing the story, if you aren't invested in the purpose of what you're doing, there's a chance that you just end up giving up on it. I actually forgot what I was doing sometimes because I simply didn't care about the objective.
Check the trophy information about the game. Only like 25-30% end up finishing the story. I think that's a direct result of the game not giving any info on the mom at all and people losing interest in finishing the game.
About the enemies, there are only a dozen types but a lot more reskins. I don't consider shield draugr different from heavy draugr. These are the same type just given a different weapon. To me, it's not enough. It gets boring after a while killing the same creatures over and over again. This is especially true if you've played as much as I did. Maybe it won't be so obvious on a normal playthrough.