Anyone else behind the “Ariana Dumbledore” was an obscurus theory? I need to reread the Deathly Hallows, but didn’t a lot of people think she was a squib, just like Grindelwald did with Credence (I guess this might be a point against the theory, since Grindy shouldn’t have assumed Credence was just a squib in that case)? Overall I think it’s a solid theory. Grindy would have realized the power of an obscurus and later put that knowledge to use by pulling a Magneto. Would also give a really solid explanation behind her mysterious death if she lost control.
As for Grindelwald’s lack of abilities, I also think you are being too hard. That amazing duel is not set to happen for another 19 years after this first Beasts movie. That’s plenty of time for him to further hone his skills and obtain the Elder Wand. And it still did take quite a force to bring him down. It is somewhat odd though I admit because it did sound like he and Dumbledore were pretty powerful in their younger days. Still, we don’t even know how good Dumbledore is at this point either. He likely has a good bit of growing to do himself.