I'm going to try to write this quick before I jinx it and E wakes up.
So, E starting sleeping through the night around one month. Like 7pm to 9am. Yay me, right? At two months we discovered she had a milk and soy intolerance, so I went on the MSPI diet. It took two whole months for it to clear her system. Suddenly, at four months and when all her symptoms had cleared she was waking every two hours again and has been that way every night since then. Let me tell you, I questioned my sanity at a few points along the road.
Now, at eight months, I am slowly adding dairy back in to test her intolerance. No reaction yet, thankfully, but suddenly BOOM she's sleeping in three hour increments and tonight she has slept for four hours straight. I'm not sure what to make of this. Any thoughts?
I attempted to sleep train a few different ways, but she could cry all night if I let her and would somehow thwart every attempt I made. Even when I found something that worked, it only worked for a week before she figured out a way around it, lol. I eventually gave up and here we are now.

Re: Coincidence?
Mine slept really well at like 2mo or something. By 3mo they hit 4mo wakeful but were sleeping 11-5. I've gotten them to 930-10 to 630- 7 but I don't know that they will ever do the super long stretches some get.
It sounds like maybe mspi/soy wasn't the issue. Here's hoping sleep continues to get better!