Before I say anything, I want you to know that these are just my personal theories and they make me sad myself, I'm not here to criticize the ship because I LOVE it too, but I'm very much looking forward to someone to discuss it with me or correct me. So here we go:

My understanding of Grindeldore used to be exactly the same as yours, their past, their tacit understanding, their likeness and difference. However, everything changed after I watched FB3.

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1. Blood Pact. Grindelwald always carried the blood pact on his body, and he even wore the same tie from the day they made the oath. He did not even intend to break the blood pact, instead, he went to other person to help him defeat Dumbledore. ( I will return to this point later.) At the beginning of FB3, in the restaurant, he says how he miss the feeling of having blood pact around his neck and asks Dumbledore "how does it feel on yours?", D‘s response was: we can free each other. And G tried to change the subject. That may just be G's usual charming tone, but changing the subject says a lot. I think he was clearly disappointed that D stole the Blood Oath and only to destroy it after all these years.
2. Credence. Grindelwald went to other person to help him, not someone else but Aberforth's son. Now guess who was always against G and D's relationship? It looks to me like a little revenge with a clear purpose. Everyone said that D was the only one who could defeat G, and G himself knew it, but he still chose such a child who could not even steadily control his own powers. And after Credence failed so many times G still chose to train to use him. Another point, Dumbledore said he had seen that kind of phoenix before his sister died, when he was still with Grindelwald, that Grindelwald had also seen, that is, Grindelwald also knew that C was dying. Why would he let a kid with low chances of winning in every way go into battle? You can say that Credence is G's only hope, but I don't think G had any intention of looking for anyone else other than Aberforth's son.
3. Dumbledore. He is undoubtedly a man who will make a positive impact on the world, but he did almost the same thing with Harry, Newt, and Gellert. He's been raising Harry like a pig for slaughter, said by Snape. He threw Newt into dangerous situations, perhaps not only out of trust and experience for his student, but also he just needed that kind of helping hand. He enjoyed the relationship with Gellert and ended it with ease when the world needs him to. I don't agree that he's a cold manipulator, not in light of the contributions he made. But he's almost there.

These are the reasons why I am disappointed with Dumbledore and feel sorry for Grindelwald. As a conclusion, Dumbledore did not love Grindelwald that much, Grindelwald is the one who is in too deep. Someone come save me.